A 15 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 rate ... 19.50 / 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 <==

Well determine the cost of one lb. To do that divide $19.50 by 15. This gives you $1.30 per lb. Multiply this by 12 1/2 to get your answer. This is $16.25