Your friend has $80 when he goes to the fair. He spends $4 to enter the fair and $12 on food. Then he has $80-$4-$12=$64 to spend on rides.
Let x be the number of rides he did. Then he spent $1.25x on all rides.
Thus, he has left over after x rides exactly $64-$1.25x=$(64-1.25x).
Answer: correct choice is C.