This question tests 1st grade ability to solve addition and subtraction word problems within 20 (CCSS.1.OA.1). This is a comparing problem with difference unknown. We compare two amounts to find how many more or fewer. The story tells us Marcus saw 12 birds and Yuki saw 9 birds. Choice C is correct because to find how many more birds Marcus saw, we subtract: 12 - 9 = 3. We can represent this as an equation with unknown: 12 - 9 = ?. Choice A is a common error where students find the total instead of the difference (12 + 9 = 21); this happens because compare problems require understanding 'how many more' as subtraction. To help students: Act out problems with physical objects; draw pictures representing the situation; teach problem type structures explicitly (adding to, taking from, putting together, taking apart, comparing); use consistent language for each type; practice writing equations with boxes for unknowns; emphasize keywords carefully ('more' can mean add or compare); model thinking aloud ('Marcus has 12, Yuki 9, need how many more for Marcus, so I subtract'); practice all unknown positions; connect to familiar experiences.

This question tests 1st grade ability to solve addition and subtraction word problems within 20 (CCSS.1.OA.1). This is an adding to problem with change unknown. We start with one amount, end with the total, and find what was added. The story tells us Chen has 6 pencils and gets some more, now having 11. Choice A is correct because to find how many pencils Chen got, we subtract: 11 - 6 = 5, or think of it as 6 + ? = 11. We can represent this as an equation with unknown: 6 + ? = 11. Choice C is a common error where students select a number from the story without solving, like picking the total 11. To help students: Act out problems with physical objects; draw pictures representing the situation; teach problem type structures explicitly (adding to, taking from, putting together, taking apart, comparing); use consistent language for each type; practice writing equations with boxes for unknowns; emphasize keywords carefully ('some more' indicates addition, but unknown is the change); model thinking aloud ('I started with 6, ended with 11, to find what was added, I can subtract or count on'); practice all unknown positions; connect to familiar experiences.

This question tests 1st grade ability to solve addition and subtraction word problems within 20 (CCSS.1.OA.1). This is a taking from problem with result unknown. We start with one amount and take away some to find what is left. The story tells us Jamal has 14 cookies and eats 6 cookies. Choice D is correct because to find how many cookies Jamal has left, we subtract: $14 - 6 = 8$. We can represent this as $14 - 6 = ?$. Choice B is a common error where students select a number from the story without solving, such as picking the amount eaten, 6. This happens because calculation facts may not be automatic yet. To help students: Act out problems with physical objects; draw pictures representing the situation; teach problem type structures explicitly (adding to, taking from, putting together, taking apart, comparing); use consistent language for each type; practice writing equations with boxes for unknowns; emphasize keywords carefully ('more' can mean add or compare); model thinking aloud ('I started with 14, took away 6, now I need to find what's left, so I subtract'); practice all unknown positions; connect to familiar experiences.

This question tests 1st grade ability to solve addition and subtraction word problems within 20 (CCSS.1.OA.1). This is an adding to problem with result unknown. We start with one amount and add more to find the total. The story tells us Keisha has 6 points and scores 7 more. Choice B is correct because to find how many points Keisha have now, we add: $6 + 7 = 13$. We can represent this as an equation with unknown: $6 + 7 = ?$. Choice A is a common error where students might add incorrectly, like $6 + 6 = 12$, miscounting. To help students: Act out problems with physical objects; draw pictures representing the situation; teach problem type structures explicitly (adding to, taking from, putting together, taking apart, comparing); use consistent language for each type; practice writing equations with boxes for unknowns; emphasize keywords carefully ('more' can mean add or compare); model thinking aloud ('I started with 6, got 7 more, now I need to find the total, so I add'); practice all unknown positions; connect to familiar experiences.

This question tests 1st grade ability to solve addition and subtraction word problems within 20 (CCSS.1.OA.1). This is a taking apart problem with one addend unknown. We start with a total and one part, and find the other part. The story tells us Maya has 18 stickers and 11 are stars. Choice A is correct because to find how many stickers are not stars, we subtract: 18 - 11 = 7. We can represent this as an equation with unknown: 18 - 11 = ?. Choice B is a common error where students add instead of subtracting (18 + 11 = 29); this happens because choosing the operation from word problem context is challenging. To help students: Act out problems with physical objects; draw pictures representing the situation; teach problem type structures explicitly (adding to, taking from, putting together, taking apart, comparing); use consistent language for each type; practice writing equations with boxes for unknowns; emphasize keywords carefully ('more' can mean add or compare); model thinking aloud ('I have 18 total, 11 are stars, need non-stars, so I subtract'); practice all unknown positions; connect to familiar experiences.

This question tests 1st grade ability to solve addition and subtraction word problems within 20 (CCSS.1.OA.1). This is an adding to problem with start unknown. We add to an unknown starting amount to reach a total, and find the start. The story tells us Carlos has some toy cars, gets 7 more, and now has 15. Choice A is correct because to find how many toy cars Carlos had at the start, we subtract: 15 - 7 = 8. We can represent this as an equation with unknown: ? + 7 = 15. Choice B is a common error where students add instead of working backwards (15 + 7 = 22); this happens because start unknown problems require reversing the typical operation. To help students: Act out problems with physical objects; draw pictures representing the situation; teach problem type structures explicitly (adding to, taking from, putting together, taking apart, comparing); use consistent language for each type; practice writing equations with boxes for unknowns; emphasize keywords carefully ('more' can mean add or compare); model thinking aloud ('Ended with 15 after adding 7, need start, so I subtract'); practice all unknown positions; connect to familiar experiences.

This question tests 1st grade ability to solve addition and subtraction word problems within 20 (CCSS.1.OA.1). This is a comparing problem with difference unknown. We compare two amounts to find how many more or fewer. The story tells us Carlos has 8 stamps and Keisha has 13 stamps. Choice B is correct because to find how many more stamps Keisha has, we subtract: 13 - 8 = 5. We can represent this as an equation with unknown: 13 - 8 = ?. Choice A is a common error where students find the total instead of the difference: 8 + 13 = 21. To help students: Act out problems with physical objects; draw pictures representing the situation; teach problem type structures explicitly (adding to, taking from, putting together, taking apart, comparing); use consistent language for each type; practice writing equations with boxes for unknowns; emphasize keywords carefully ('how many more' means subtract to compare); model thinking aloud ('Keisha has 13, Carlos has 8, to find how many more, I subtract'); practice all unknown positions; connect to familiar experiences.

This question tests 1st grade ability to solve addition and subtraction word problems within 20 (CCSS.1.OA.1). This is a taking from problem with start unknown. We subtract from an unknown starting amount to reach a remainder, finding the start by adding. The story tells us Emma has some books, gives away 5, now has 9 left. Choice B is correct because to find how many books Emma had at the start, we add: 9 + 5 = 14, or ? - 5 = 9. We can represent this as ? - 5 = 9. Choice A is a common error where students subtract instead, getting 9 - 5 = 4. This happens because start unknown requires working backwards. To help students: Act out problems with physical objects; draw pictures representing the situation; teach problem type structures explicitly (adding to, taking from, putting together, taking apart, comparing); use consistent language for each type; practice writing equations with boxes for unknowns; emphasize keywords carefully ('at the start' with subtraction and remainder suggests finding initial amount); model thinking aloud ('Ended with 9 after giving 5, so start was? I add back'); practice all unknown positions; connect to familiar experiences.

This question tests 1st grade ability to solve addition and subtraction word problems within 20 (CCSS.1.OA.1). This is an adding to problem with change unknown. We start with one amount, end with another, and find what was added. The story tells us Sofia has 9 crayons and now has 15 crayons after getting some more. Choice B is correct because to find how many crayons Sofia got, we subtract: $15 - 9 = 6$, or add up: $9 + 6 = 15$. We can represent this as $9 + ? = 15$. Choice D is a common error where students calculate incorrectly, such as subtracting smaller from larger in the wrong order like $9 - 4 = 5$, but actually it's $15 - 9$ or similar miscount. This happens because in change unknown problems, students don't recognize they need to work backwards. To help students: Act out problems with physical objects; draw pictures representing the situation; teach problem type structures explicitly (adding to, taking from, putting together, taking apart, comparing); use consistent language for each type; practice writing equations with boxes for unknowns; emphasize keywords carefully ('more' can mean add or compare); model thinking aloud ('I started with 9, ended with 15, need to find what was added, so I subtract or add up'); practice all unknown positions; connect to familiar experiences.

This question tests 1st grade ability to solve addition and subtraction word problems within 20 (CCSS.1.OA.1). This is a putting together problem with addend unknown, or taking apart with part unknown. We know the total and one part, and find the other part. The story tells us Marcus has 18 stamps, of which 10 are animals. Choice C is correct because to find how many stamps are not animals, we subtract: $18 - 10 = 8$, or add up: $10 + 8 = 18$. We can represent this as $10 + ? = 18$. Choice B is a common error where students select a number from the story without solving, such as picking 10. This happens because calculation facts may not be automatic yet. To help students: Act out problems with physical objects; draw pictures representing the situation; teach problem type structures explicitly (adding to, taking from, putting together, taking apart, comparing); use consistent language for each type; practice writing equations with boxes for unknowns; emphasize keywords carefully ('more' can mean add or compare); model thinking aloud ('I have 18 total, 10 are animals, need the non-animals, so I subtract'); practice all unknown positions; connect to familiar experiences.