**Answer:** $100.

The solution can be found using algebra, and calling the original total T.

I originally spent 1/5 of T, leaving 4/5 of T.

I then spend 1/5 of (4/5 of T).

Therefore in total I spent 1/5 of T plus 1/5 of (4/5 of T), and this equalled $36.00. Writing this more mathematically gives:

T 1 4

- + - x - x T = 36

5 5 5

Multiplying throughout by 25 gives:

5T + 4T = 900

9T = 900

T = 100. QED.