Add 12/15 and 81/21, Solve 12/15 + 3 18/21 as a fraction

Add 12/15 and 81/21

1^st number: 12/15, 2^nd number: 3 18/21

12/15 + 81/21 is 163/35.

Find the least common denominator or LCM of the two denominators:
LCM of 15 and 21 is 105

Next, find the equivalent fraction of both fractional numbers with denominator 105
For the 1st fraction, since 15 × 7 = 105,
12/15 = 12 × 7/15 × 7 = 84/105
Likewise, for the 2nd fraction, since 21 × 5 = 105,
81/21 = 81 × 5/21 × 5 = 405/105
Add the two like fractions:
84/105 + 405/105 = 84 + 405/105 = 489/105
489/105 simplified gives 163/35
So, 12/15 + 81/21 = 163/35
In mixed form: 423/35

MathStep (Works offline)

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