A stationery shop's monthly sales revenue went up by 20% while the number of items it sold in the month went down by 4%. The average price per item therefore went up by what percent?

Revenue = number of items × average price, so the price factor is 1.20/0.96 = 1.25, a rise of 25%. Adding 20 and 4 to get 24% treats the two changes as though they simply combined by addition. Multiplying the factors $(1.20\times 0.96=1.152$, i.e. 15.2%) is the wrong operation altogether — price is revenue DIVIDED by quantity, so the fall in quantity has to be divided out, which pushes the price up rather than down. Subtracting the two figures gives 16%, also wrong.