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To get the correct answer. If you were adding 1/5 and 1/7, you might not know that the answer is 12/35 unless you knew that 1/5 = 7/35 and 1/7 = 5/35

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Q: Why do you need a common denominator to add and subtract fractions?
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What are fraction that have same denominator called?

They are called like fractions.In order to add or subtract fractions you will usually need to convert them to like fractions. Then you simply add or subtract the numerators (as required) to form the numerator of the answer. The same (or common) denominator is the denominator of the answer. You may need to simplify the resulting fraction.


What is the common denominator for 1 over 3 and 4 over 9?

Well, darling, the common denominator for 1/3 and 4/9 is 9. You see, to add or subtract fractions, you need a common denominator, and in this case, multiplying 3 by 3 gives you 9. So, there you have it, simple math for a simple question.


What are compatible numbers for fractions?

I am not entirely sure what you mean, but if you need to add, subtract, or compare two fractions, they need to have the same denominator.


How can you tell whether the sum of two fractions with a common denominator is greater than 1 less than 1 or equal to?

To determine whether the sum of two fractions with a common denominator is greater than, less than, or equal to 1, you need to add the numerators of the fractions together and compare the result to the common denominator. If the sum of the numerators is greater than the denominator, the sum of the fractions will be greater than 1. If the sum of the numerators is less than the denominator, the sum of the fractions will be less than 1. If the sum of the numerators is equal to the denominator, the sum of the fractions will be equal to 1.


What is 11 over 12 minus 3 over 8?

Well, let's take a moment to appreciate the beauty of these fractions. To subtract them, we need a common denominator. In this case, it's 24. So, 11/12 becomes 22/24, and 3/8 becomes 9/24. Subtracted, they give us 13/24, a lovely little fraction full of possibilities.