Find out primes of each denominator:

Add 1/15 and 7/10

Solution:

Step 1:            Find out primes of each denominator.

15 = 5 x 3

10 = 5 x 2

Step 2:            In the given example, 15 is the largest denominator, so use the 5 and the 3; now look at the second denominator's primes-here the five already appears in the list, but the 2 does not, so use the 2.

5 x 3 x 2 = 30

Step 3:            Rewrite with least general denominators.

1/15 = 2/30

7/10 = 21/30

Step 4:            Add the new fractions.

2/30 + 21/30 = 23/30