You need to know the least common denominator (LCD) of 7 and 21 if you want to add or subtract two fractions with 7 and 21 as denominators.
The least common denominator, also called lowest common denominator (LCD), of 7 and 21 is 21.
Here is a math problem example where you need to know the LCD of 7 and 21 to solve:
3/7 + 2/21 = ?
Step 1) Take the LCD and divide each denominator by it as follows:
21/7 = 3
21/21 = 1
Step 2) Multiply each nominator with the respective answers from Step 1:
3 x 3 = 9
2 x 1 = 2
Step 3) Put it all together to solve the problem:
9/21 + 2/21 = 11/21
= 3/7 + 2/21 = 11/21
It's that easy! Once again, the lowest common denominator (LCD) of 7 and 21 is as follows:
21
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