[followed up here]
Let's push this forward. The question is "how much cash?" The reference point is CoinCalc.
Your challenge is to outline the supporting materials so that this activity will a) scale from easy to hard, b) throw a few curveballs at the students who figure out its mechanics quickly, and c) offer visual confirmation of the answer to provoke a discussion of sources of error.
If you then consider the fact that a) it's easier to mix coins than unmix them and b) it's easier to tally the value of a roll of coins than a pile of loose change, you'll understand why producing this unit took a week of detailed planning and an afternoon of careful shooting.