To find a common denominator:
Think of the denominators 4 and 8 in 3/4 and 5/8 . Does the smaller denominator 4 divide evenly into the larger 8? Yes, then the larger denominator 8 is the common denominator.

If the smaller denominator does not divide evenly into the larger, Multiply the larger denominator by 2, 3, and then 4, etc. Each time check for division by the smaller denominator.

In the fractions 3/4 and 2/3:

1. Multiply the larger denominator 4 by 2 to get 8. Does 3 divide evenly into 8? No.

2. Multiply the larger denominator 4 by 3 to get 12.  Does 3 divide evenly into12? Yes, so 12 is the least common denominator of the denominators 4 and 3.