The speed of the current is 4 km/hr. A boat goes 10 km upstream and comes back to the starting point in 80 minutes. What is the speed of the boat in still water?

Let boat speed = b km/hr. Upstream speed = (b-4), downstream = (b+4). Total time = 80 minutes = 4/3 hours. So 10/(b-4) + 10/(b+4) = 4/3. Cross-multiply: 30(b+4) + 30(b-4) = 4(b²-16). This gives 60b = 4b² - 64, or 4b² - 60b - 64 = 0. Dividing by 4: b² - 15b - 16 = 0, giving (b-16)(b+1) = 0. Thus b = 16 km/hr. Option C (18) could result from sign errors in the quadratic.