Subtract 20/21 from 30/40, Solve 30/40

Subtract 20/21 from 30/40

30/40 - 20/21 is -17/84.

Find the least common denominator or LCM of the two denominators:
LCM of 40 and 21 is 840

Next, find the equivalent fraction of both fractional numbers with denominator 840
For the 1st fraction, since 40 × 21 = 840,
30/40 = 30 × 21/40 × 21 = 630/840
Likewise, for the 2nd fraction, since 21 × 40 = 840,
20/21 = 20 × 40/21 × 40 = 800/840
Subtract the two like fractions:
630/840 - 800/840 = 630 - 800/840 = -170/840
After reducing the fraction, the answer is -17/84

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