Solution:

In still water, total distance is 20 km in 2 hours, so boat speed \(v_b = 10\text{ km/h}\). Water velocity \(v_w = 5\text{ km/h}\). Upstream speed \(v_b - v_w = 5\text{ km/h}\). Time upstream = \(10/5 = 2\) hours. Downstream speed \(v_b + v_w = 15\text{ km/h}\). Time downstream = \(10/15 = 2/3\) hours. Total time = \(2 + 2/3 = 8/3\) hours = \(160\) minutes.