A shopkeeper's stock of 800 notebooks rose to 920 after a fresh supply arrived. By what per cent did the supply raise the stock, and what per cent of the new stock do the added notebooks form?

120 notebooks were added. A rise is always measured on the earlier stock: 120/800 = 0.15 = 15%. A share of the new stock is measured on the new stock: 120/920 = 0.1304 = 13.04%. One gap, two bases, two different percentages - which is exactly why '15% and 15%' is wrong. The third option swaps the two answers, and 86.96% is 800/920, the share of the new stock that was already there.