A boat goes 72 km. and returns on starting point in 24 hours. If the ratio of speed of boat in still water and speed of stream is 2:1 then find the speed of boat in still water. एक नाव 72 किमी. जाकर 24 घंटें में पुनः अपने स्थान पर वापस आ जाती है। यदि शान्त जल में नाव की चाल तथा धारा की चाल का अनुपात 2 : 1 है तो शान्त जल में नाव की चाल ज्ञात कीजिए।

Let boat speed in still water be 2x and stream speed be x. Downstream speed = 2x + x = 3x. Upstream speed = 2x - x = x. Total time = 72/(3x) + 72/x = 24/x + 72/x = 96/x = 24 hours. Solving gives x = 4 km/h. Boat speed in still water = 2x = 8 km/h. Verification: downstream speed = 12 km/h (72/12 = 6 hours), upstream speed = 4 km/h (72/4 = 18 hours), total = 24 hours.