Subtract 90/18 from 72/21, Solve 3 9/21

Subtract 90/18 from 72/21

1^st number: 3 9/21, 2^nd number: 5 0/18

72/21 - 90/18 is -11/7.

Find the least common denominator or LCM of the two denominators:
LCM of 21 and 18 is 126

Next, find the equivalent fraction of both fractional numbers with denominator 126
For the 1st fraction, since 21 × 6 = 126,
72/21 = 72 × 6/21 × 6 = 432/126
Likewise, for the 2nd fraction, since 18 × 7 = 126,
90/18 = 90 × 7/18 × 7 = 630/126
Subtract the two like fractions:
432/126 - 630/126 = 432 - 630/126 = -198/126
After reducing the fraction, the answer is -11/7

MathStep (Works offline)

Download our mobile app and learn to work with fractions in your own time:
Android and iPhone/ iPad