Find the baby's weight increase?(need help quickly before thursday)

Question

A baby weighed 6 lb 11 oz at birth. At 6 months of age, the baby weighed 15 lb 9 oz. Find the baby's increase in weight during the 6 months.

Answer 1

Subtract the birth weight from the 6 month weight.

15 lb   9 oz

 -6 lb 11 oz

The problem is that you can't subtract 11 from 9.

No problem! "Borrow" a pound (16 oz) from 15 and add it to the ounces.

14 lb 25 oz

 -6 lb 11 oz

  8 lb 14 oz