Scott knew that the only chance we would have to answer this problem would be if he kept his money in a bank that pays no interest, so that's what he did. The bank simply stored his money in a mayonnaise jar under a tightly-guarded mattress, and he was allowed to come in and take some out of it each week.
He started with $500. He withdrew (500 - 20) = $480 in 12 weeks.
So he must have come in and picked up 480/12 = $40 each week.
