Find GCF and LCM
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6th Grade Math › Find GCF and LCM
A student claims: “The LCM of 6 and 8 is $6\times 8=48$.” Which statement is correct?
Incorrect; the LCM must be smaller than both numbers, so it is 4.
Incorrect; the LCM is 24 because it is the smallest common multiple of 6 and 8.
Correct; the LCM is always the product of the two numbers.
Correct; 48 is the smallest multiple of both 6 and 8.
Explanation
This question tests correcting a misconception about LCM, not always the product. LCM is smallest common multiple, like LCM(6,8)=24, not 48. Student said 48, a multiple but not least; correct LCM=24 from multiples. Statement B is correct: incorrect, LCM is 24. Errors: thinking product always LCM (only if coprime), or LCM smaller than numbers. Find LCM by multiples list or primes: 6=2*3, $8=2^3$, $LCM=2^3$*3=24. Applications: timing like bus schedules.
Find the greatest common factor of 54 and 72 by using prime factorization.
Which choice correctly gives the prime factorizations and the GCF?
$54=2\cdot 3^3$, $72=2^3\cdot 3^2$, so $\text{GCF}=2^3\cdot 3^3=216$
$54=2\cdot 3^3$, $72=2^3\cdot 3^2$, so $\text{GCF}=2\cdot 3^2=18$
$54=2\cdot 3^2$, $72=2^3\cdot 3^2$, so $\text{GCF}=2\cdot 3^2=18$
$54=2\cdot 3^3$, $72=2^2\cdot 3^2$, so $\text{GCF}=2^2\cdot 3^2=36$
Explanation
This question tests finding the GCF of 54 and 72 using prime factorization, where GCF is the product of the lowest powers of common primes, helping in simplifying fractions or dividing evenly. The GCF is the largest number dividing both, for example, GCF(36,48)=12; LCM uses highest powers, like LCM(6,8)=24 from $2^3$3. For $54=23^3$ and $72=2^3$$*3^2$, the $GCF=2^1$$3^2$=29=18, as it takes the minimum exponents. The correct choice is the one with accurate factorizations and GCF=18. A common error is using highest exponents like for LCM, getting $2^3$$3^3$=216, or wrong factorization like $54=23^2$ (which is 18, not 54). To find GCF with primes: factor each number, take common primes with lowest powers, multiply; for LCM, use highest powers. Applications include reducing ratios; mistakes include incorrect exponents, confusing GCF with LCM, or arithmetic errors in multiplication.
Two students start jogging at the same time. One completes a lap every 10 minutes and the other completes a lap every 12 minutes. If they start together, after how many minutes will they both be at the starting line together again? (Find $\text{LCM}(10,12)$.)
20
22
60
120
Explanation
This question tests finding the LCM of 10 and 12, the smallest time when joggers meet again at the start, aligning their lap cycles. The LCM is the smallest number both divide into, like LCM(6,8)=24; GCF is the largest dividing both, like GCF(36,48)=12. For example, multiples of 10: {10,20,30,40,50,60,...}, of 12: {12,24,36,48,60,...}, common: {60,120,...}, least=60. They meet after 60 minutes, as smaller like 20 isn't a multiple of 12. A common error is choosing GCF=2, or a non-multiple like 22, or product 120 which isn't least. To find LCM: list multiples, find common, choose smallest; or primes: 10=25, $12=2^2$3, $LCM=2^2$35=435=60. Applications include cycle alignment; mistakes include not picking least or listing errors.
Find the least common multiple of 9 and 12. (Think of the smallest positive number that is a multiple of both.)
18
21
36
108
Explanation
This question tests finding LCM of 9 and 12, the smallest positive multiple of both. LCM is smallest both divide into, like LCM(6,8)=24 from multiples. For 9 and 12, multiples of 9={9,18,27,36,...}, 12={12,24,36,...}, common start at 36, so LCM=36. The correct answer is 36, choice C. Errors include picking GCF 3, or a larger multiple like 108, or arithmetic like 9+12=21. To find LCM: list multiples, find common, choose least; or prime factors $9=3^2$, $12=2^2$*3, $LCM=2^2$$*3^2$=36. Applications: cycles coinciding, like events every 9 and 12 days.
A student says: “The GCF of 36 and 48 is 6 because 6 is a common factor.” Which statement is correct?
The student is incorrect; the GCF must be the smallest common factor, which is 1.
The student is correct because 6 is the only common factor.
The student is incorrect; the GCF must be the largest common factor, which is 12.
The student is correct because any common factor is the GCF.
Explanation
This question tests understanding GCF as the greatest common factor, not just any, addressing a student's misconception. GCF is the largest dividing both, like GCF(36,48)=12 from common factors {1,2,3,4,6,12}. The student picked 6, a common factor, but not the greatest, so incorrect; the largest is 12. The correct statement is B, that the student is incorrect and GCF is 12. Errors include thinking any common factor is GCF, or confusing with smallest like 1, or believing 6 is only one. To find GCF, list factors, find common, pick largest; prime factors use lowest powers. This clarifies definitions for applications like grouping.
Use the distributive property to factor the sum $36+8$ using the greatest common factor. Which expression is correct and fully reduced (the numbers inside the parentheses have no common factor greater than 1)?
$4(9+2)$
$12(3+1)$
$8(4+1)$
$2(18+4)$
Explanation
This question tests using the distributive property to factor a sum like 36+8 using the GCF, where you factor out the greatest common factor and ensure the quotients inside are coprime, meaning they have no common factor greater than 1. The GCF is the largest number dividing both, like GCF(36,48)=12 from common factors {1,2,3,4,6,12}; LCM is the smallest both divide into, like LCM(6,8)=24. For example, to factor 36+8, GCF(36,8)=4, so 4(36/4 + 8/4)=4(9+2), and 9 and 2 are coprime since their factors are {1,3,9} and {1,2} with only 1 in common. The correct expression is 4(9+2), as it's using the GCF and fully reduced. A common error is factoring with a smaller number like 2(18+4), where 18 and 4 share a factor of 2, or using a non-factor like 8(4+1) but 8 doesn't divide 36 evenly since 36/8=4.5. To factor using GCF: (1) find GCF(a,b), (2) divide both by GCF, (3) write GCF(quotient1 + quotient2), (4) verify quotients are coprime. Applications include simplifying expressions; mistakes include not using the greatest factor, leaving common factors inside, or arithmetic errors like wrong division.
Two hallway lights flash on a timer. One flashes every 6 seconds and the other flashes every 8 seconds. If they flash together now, after how many seconds will they flash together again (the LCM of 6 and 8)?
14
24
48
56
Explanation
This question tests finding the LCM, the least common multiple or smallest number both divide into, like when events coincide again. LCM is the smallest number both divide into, for example, LCM(6,8) from multiples of 6={6,12,18,24,...} and 8={8,16,24,...}, common={24,48,...}, least=24. For this timer problem, list multiples to find the first common one at 24 seconds. The correct answer is 24, choice B, as that's when they flash together next. Errors include picking the product 48, which is a multiple but not the least, or a common factor like 2 instead. To find LCM, list multiples of each, find common multiples, and choose the smallest. Prime factorization helps: for LCM take highest powers of all primes; applications are scheduling like these lights.
A coach has 45 orange cones and 30 yellow cones. She wants to set up identical stations using all the cones with none left over. What is the greatest number of stations she can make (the GCF of 45 and 30)?
5
10
15
30
Explanation
This question tests GCF of 45 and 30 for making equal stations with all cones. GCF is largest dividing both, like GCF(36,48)=12. Factors of 45={1,3,5,9,15,45}, 30={1,2,3,5,6,10,15,30}, common={1,3,5,15}, greatest=15. Correct is 15 stations, choice C, with 45/15=3 orange and 30/15=2 yellow each. Error: picking smaller like 5, not greatest, or LCM 90 too big. Find GCF by listing factors, common, largest; or primes $45=3^2$5, 30=235, GCF=35=15. Use for equal divisions like here.
A student lists factors to find the GCF of 24 and 60:
- Factors of 24: ${1,2,3,4,6,8,12,24}$
- Factors of 60: ${1,2,3,4,5,6,10,12,15,20,30,60}$ What is the GCF of 24 and 60?
6
8
12
24
Explanation
This question tests identifying GCF from given factor lists of 24 and 60. GCF is largest common factor, like from lists for 36 and 48 yielding 12. Given lists: common={1,2,3,4,6,12}, so greatest=12. Correct is 12, choice C. Error: picking smaller like 6, not greatest, or non-common like 8. Find GCF by comparing lists, pick largest common. Primes: $24=2^3$*3, $60=2^2$35, $GCF=2^2$*3=12; used for divisions.
A science class observes a flashing light every 9 seconds and a different flashing light every 12 seconds. If they flash together at time 0, when is the next time they flash together? (Find $\text{LCM}(9,12)$.)
18 seconds
108 seconds
21 seconds
36 seconds
Explanation
This question tests finding the LCM, which is the least common multiple or smallest multiple of both 9 and 12, representing the next time lights flash together. The LCM is the smallest both divide into, for example, LCM(9,12): multiples of 9 are 9,18,27,36,... and of 12 are 12,24,36,..., common 36,72,... least 36. In contrast, GCF is largest dividing both, but here LCM for timing. For example, LCM(9,12): list multiples, first common=36. The correct answer is 36 seconds. A common mistake is smaller common like 18 (not multiple of 12) or larger like 108. To find LCM: list multiples, pick smallest common; or prime: $9=3^2$, $12=2^2$*3, $LCM=2^2$$*3^2$=36; applies to events coinciding like lights or schedules.