Tanvi and Usha push off together from the same end of a 60 m pool lane, swimming at 2 m/s and 1 m/s and turning instantly at each wall. After how many seconds do they meet for the second time?

Tanvi needs 30 s per length, Usha 60 s. First meeting: Tanvi turns at 30 s and meets Usha head-on when their total distance is $2\times 60:\frac{120}{3}=40$ s. From 60 s Tanvi swims out again $(2t-120)$ while Usha is coming back $(120-t):2t-120=120-t\Rightarrow t=80$ s, 40 m from the start. (120 s is when both are next at the start wall.)