SAT Math : How to find the lowest / least common denominator

Study concepts, example questions & explanations for SAT Math

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Example Questions

Example Question #1 : How To Find The Lowest / Least Common Denominator

3/5 + 4/7 – 1/3 = 

Possible Answers:

88/105

7/9

4/3

3/37

72/89

Correct answer:

88/105

Explanation:

We need to find a common denominator to add and subtract these fractions. Let's do the addition first. The lowest common denominator of 5 and 7 is 5 * 7 = 35, so 3/5 + 4/7 = 21/35 + 20/35 = 41/35. 

Now to the subtraction. The lowest common denominator of 35 and 3 is 35 * 3 = 105, so altogether, 3/5 + 4/7 – 1/3 = 41/35 – 1/3 = 123/105 – 35/105 = 88/105. This does not simplify and is therefore the correct answer.

Example Question #2 : How To Find The Lowest / Least Common Denominator

What is the lowest common denominator of the fractions below? 

Possible Answers:

Correct answer:

Explanation:

In order to find the lowest common denominator, you must first list the multiples of each of the denominators of the three fractions:

The lowest common denominator between these three sets of multiples is

Example Question #2 : How To Find The Lowest / Least Common Denominator

What is the Lowest Common Denominator of  ,and ?

Possible Answers:

Correct answer:

Explanation:

The smallest number they all have in common in .

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