Of the candidates at a skill centre, 24% cleared both the written test and the practical test. If 40% of those who cleared the written test also cleared the practical test, what per cent of the candidates cleared the written test?

Let $W$ be the per cent who cleared the written test. Those who cleared both are 40% of $W$, so $0.40W=24$, giving $W=24/0.40=60$%. Check: 40% of 60 = 24. 9.6% is 40% of 24 - the chain run in the wrong direction, taking the rate on the answer instead of on the unknown. 16% and 64% subtract and add the two figures, but 24 and 40 sit on different bases and cannot be combined that way.