Answer: If the number of bills counted is odd, the ones digit is 5; if the number of bills is even, the ones digit is 0.
Explanation
1) Simulate the counting of the bills:
number of bills counted value ones digit
1 5 5
2 5 + 5 = 10 0
3 10 + 5 = 15 5
4 15 + 5 = 20 0
5 20 + 5 = 25 5
6 25 + 5 = 30 0
2) Pattern: you can see that the ones digit alreternate: 5, 0, 5, 0, 5, 0, ...
If the number of bills counted is odd the ones digit is 5, if the number of bills is even the ones digit is 0.
