Hmmm- well, I suggested: Start with the biggest coins. So if you had to make 91 cents- let's say, a fifty cent piece and 25 cent piece; that's 75 cents, so you need 16 more. How do you make 16? A dime, a nickel,a peny. You could do 3 nickels and a penny but that's more coins, not fewer.
I've always been good at problem solving, LOL. Anyway... some of the kids just picked it up right away... (my son did at least).