Two masons, Iqbal and Prakash, work at daily outputs in the ratio 7 : 5. Together they plaster a compound wall in 35 days. Working on his own, how long would Prakash take?

Let the daily outputs be 7$k$ and 5$k$. Together $12k=1/35$, so $k=1/420$. Prakash's rate is 5/420 = 1/84, so he needs 84 days. 60 days is Iqbal's solo figure (7/420 = 1/60) — the faster man, not the slower. 49 days comes from scaling the joint 35 days by 7/5 as though the ratio applied to times; it applies to outputs, so the times are in the inverse ratio. 25 days is the same scaling turned the other way, $35\times 5/7$, and is impossible anyway: no one alone can beat the pair's 35 days.