Take the biggest coin that fits, then the next, then the next. It is what everyone does, and in a shop whose coins are 1c, 4c and 6c it is wrong: 8c is 6+1+1 that way, and 4+4 in fact. Finding the gap is the question.
The answer key prints on a separate page.
Real money is built so the obvious method always works. Pounds, dollars and euros are all designed that way — 1, 5, 10, 25 and 1, 2, 5, 10 both reward taking the largest coin first, every single time. That is why the rule feels like arithmetic rather than a guess, and why a shop where it fails is worth a page.
On every question here the everyday way of paying costs more coins than the best way — that gap is the question. The answer key prints both tallies side by side, because a child who answered four coins was not careless: they used a rule that usually works, and what they need to see is which rule to distrust.
One question from a sheet like this one, worked a step at a time — with a sentence saying what just happened and why.
Fewest coins, step by step