Question 64995
A baby weighs 7 1/8 pounds at birth. After four months, the baby weighs 15 2/3 pounds. How much weight did the baby gain?
Subtract the birth weight from the weight after four months.  In order to subtract fractions you need a common denominator.  In this case 3 and 8 have no common factors so the LCD is 3*8=24.  Whatever you do to the denominator to get that you also have to do to the numerator:
15 2/3 - 7 1/8 
15 2*8/(3*8) - 7 1*3/(8*3)
15 16/24 - 7 3/24  Subtract the whole number and fractions.
(15-7) (16-3)/24
8 13/24 pounds
Happy Calculating!!!