I spent 3/4 of my money and then I spent 1/5 of what was left. What part of all my money do I have left?

After spending 3/4 you have 1/4 left = 5/20.
1/5 of 5/20 is 1/20. 5/20 - 1/20 = 4/20 = 1/5.
Therefore you have 1/5 left.