Change from £5

Here's a simple real-life problem for upper primary school children. I had to solve it rapidly last week when we were in Rochester, Kent. Our drive back to Norwich would involve paying the toll of £1.50 at the Dartford crossing, for which having the exact amount is a distinct advantage. I had only a £5 note – no change whatsoever.

So, I went into a post office and used my £5 note to buy some first class stamps for letters, which I knew cost 41p each. The mathematical problem was this: how many stamps to buy to guarantee that the change would include the £1.50 I needed?