You need to know the least common denominator (LCD) of 4 and 21 if you want to add or subtract two fractions with 4 and 21 as denominators.
The least common denominator, also called lowest common denominator (LCD), of 4 and 21 is 84.
Here is a math problem example where you need to know the LCD of 4 and 21 to solve:
3/4 + 2/21 = ?
Step 1) Take the LCD and divide each denominator by it as follows:
84/4 = 21
84/21 = 4
Step 2) Multiply each nominator with the respective answers from Step 1:
3 x 21 = 63
2 x 4 = 8
Step 3) Put it all together to solve the problem:
63/84 + 8/84 = 71/84
= 3/4 + 2/21 = 71/84
It's that easy! Once again, the lowest common denominator (LCD) of 4 and 21 is as follows:
84
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