Answer:
I am not too sure so I deeply apologize for any wrongs, but this is my thinking.
Joe bought each taco for 4 dollars, and bought an asada fry for 50 cents.
On the other hand-
Sam also bought 1 taco for 12 dollars, and bought 2 asada fries, one of them was 1 dollar, and one of them was 75 cents.
