Alejandro saved some money for his trip to a water park. he spent 2/3 of the money on rides and was left with $27.50.

If he spent 2/3 of the money on rides, he has 1/3 of the original amount of money remaining. I am assuming you are looking for the amount in the beginning.

1/3 = 27.5/x

x is the total amount at the beginning

1 (27.5) = 27.5

3 (27.5) = x

x = 82.5

He had $82.50 at the start.