Question 178083
There are probably several ways to think about this problem but in an emergency, just trying to write them out is never a bad thing.  To show what I mean, I will call the number of $5's, f,the number of $10's t, and the number of $20's w.
If w=1, what can f and t be?
If w=1 and f=1, what is t?  That doesn't work.
If w=1 and f=2, what is t? It is 2.
And so on and so on.