Your mistake is that 1 pound and 15 ounces does not equal 1.15 pounds. You also did not make everything into the same measurement (you mixed up pounds and ounces). I will make everything into the measurement of ounces, since the numbers do not come out clean if we do pounds.
18 pounds (original) = 288
1 pound 15 ounces = 31 ounces
3 1/8 pound = 50 ounces
4 1/2 pound = 72 ounces
So after cutting out those steaks, Mr. Rand has 288-31-50-72 = 135 ounces. Then, it states that he divides those up into 9 ounce patties:
135/9 = 15 patties
And sells them at 3 for $3.12. There are 5 3's in 15 patties, so it is 5 * $3.12 = Mr Rand receives $15.60 for the patties.