You should change the order of ifs in the case 100

AHHHH I see now. Sorry, I had a mental block on this one. Thank you.

if someone hands over $100 for a $25 ticket, they've handed you the $75 change they're due

What?? They handed you 100, not 75. And you're supposed to give out change, not the customer. You can't give back change of 75 when you only have 25 from first customer.

No, they pay you with a 100 dollars bill. And you only have the 25 dollars bill of the previous client.

Test.assertEquals(tickets([25, 100]), "NO");

...if someone hands over $100 for a $25 ticket, they've handed you the $75 change they're due. There would be no need to make change from the drawer.

RIGHT??

When posting code you should mark your message with spoiler code.

You can give back 3 25s when given a 100 dollar bill. If you actually have the bills.