I believe something like this was included as a filler in one of the MAA journals many years ago. I am searching for the exact reference (for the filler, or an earlier source).
Someone dies, and his will makes some bequests, then ends by saying "after taxes are paid, any remaining money should be donated to charity". Well, if charitible donations are tax-deductible, then the amount of the donation effects the taxes. What to do? The I.R.S. supposedly has a form where you do this computation, but it amounts essentially to trial and error. In fact the problem can be solved by high-school algebra. (Solution of a linear equation.) Or, not even that: the problem can be solved using the "method of false position": that method is described in the Rhind Papyrus, which dates from maybe 2650 B.C.
And some people say the I.R.S. is behind the times...