f(x) = 1.25x + 12.
12 Is the cost of food, and 1.25x represents that every ride (x) is 1.25 dollars.
f(1) = 1.25(1) + 12
f(1) 13.25
If he rides one ride, then it will cost this 13.25 dollars in total
f(2) = 1.25(2) + 12
f(2) = 14.5
If he rides two rides, then it will cost 14.5 dollars in total
f(3) = 1.25(3) + 12
f(3) = 15.75
If he rides 3 rides, it will cost 15.75 dollars in total, etc. etc.
Hope this helps!!