A boat that rows at 12 km/h in still water takes twice as long to go 40 km upstream as it takes to come back the same 40 km downstream. The complete round trip takes how many hours?

Twice the time over the same distance means half the speed, so 12 − s = (12 + s)/2 → 24 − 2s = 12 + s → s = 4 km/h. Downstream at 16 km/h takes 40/16 = 2.5 h and upstream at 8 km/h takes 40/8 = 5 h, a total of 7.5 hours. (Ignoring the current altogether gives 80/12 = 6.67 h.)