A15 lb. costs $19.50. at that same rate, how much would 12 1/2 lb. ham cost?

There are 2 ways to do this

(1) find the unit rate19.50 / 15 = 1.30 per lb.
for 12 1/2 (or 12.5) lbs = 1.30 * 12.5 = 16.25 <===

(2) proportion...
15 / 19.50 = 12.5 / x...15 lbs to 19.50 = 12.5 lbs to x
 cross multiply
 (15)(x) = (19.5)(12.5)
  15x = 243.75
 x = 243.75 / 15
x = 16.25 <==

The amount used= 13.04- 1.9 = 11.14