$25 is the total amount of money. Admission is a constant, meaning that price will not vary and is fixed. The rides each cost something so the price of rides will vary, unlike the constant (admission).
Thus making x the number of rides, we have the equation: 25 = 4 + 1.5x. We need to solve for x, but x must be a whole number so we will round down if the solution is not an integer.
