A boat's still-water speed is 5 m/s and the current is 3.6 km/h. How long does it take to go 4.5 km downstream and return?

5 m/s = 18 km/h. Downstream 21.6, upstream 14.4 km/h. 4.5/21.6 h = 12.5 min, 4.5/14.4 h = 18.75 min. Total = 31.25 min = 31 min 15 s. (30 min ignores the current.)