You need to know the least common denominator (LCD) of 4 and 5 if you want to add or subtract two fractions with 4 and 5 as denominators.
The least common denominator, also called lowest common denominator (LCD), of 4 and 5 is 20.
Here is a math problem example where you need to know the LCD of 4 and 5 to solve:
3/4 + 2/5 = ?
Step 1) Take the LCD and divide each denominator by it as follows:
20/4 = 5
20/5 = 4
Step 2) Multiply each nominator with the respective answers from Step 1:
3 x 5 = 15
2 x 4 = 8
Step 3) Put it all together to solve the problem:
15/20 + 8/20 = 23/20
= 3/4 + 2/5 = 23/20
It's that easy! Once again, the lowest common denominator (LCD) of 4 and 5 is as follows:
20
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