Although it is true that the problem would be simpler if one had to take in to account only the total amount of money the clerk has and not its distribution across the different notes, the statement that you could always give change if this condition wasn't in place is incorrect, as the second sample test shows. Generally speaking, I reckon that more detailed instructions for some kata would be beneficial.

Hi, I'm working on this kata with some students but I cannot wrap my hear around it mathematically.
You say that the 78 is the right result for 101, but why?
Why haven't you gon any number larger than 18?
Thanks :)