A workshop raised its monthly output by 40% in April, and then cut the April output by 25% in May. The May output is what per cent more than the March output?

Take the March output as 100. April = $100\times 1.40=140$, and May = $140\times 0.75=105$. So May stands 5% above March. 15% is the classic error of adding the two changes (40 - 25); successive changes multiply, and the 25% cut bites on 140, not on 100. 30% cuts only the 40% rise by a quarter, and 35% reads the fall of 35 units from 140 as if it were a percentage.