You need to know the least common denominator (LCD) of 21 and 4 if you want to add or subtract two fractions with 21 and 4 as denominators.
The least common denominator, also called lowest common denominator (LCD), of 21 and 4 is 84.
Here is a math problem example where you need to know the LCD of 21 and 4 to solve:
3/21 + 2/4 = ?
Step 1) Take the LCD and divide each denominator by it as follows:
84/21 = 4
84/4 = 21
Step 2) Multiply each nominator with the respective answers from Step 1:
3 x 4 = 12
2 x 21 = 42
Step 3) Put it all together to solve the problem:
12/84 + 42/84 = 54/84
= 3/21 + 2/4 = 54/84
It's that easy! Once again, the lowest common denominator (LCD) of 21 and 4 is as follows:
84
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