A boat goes 80 km upstream in 16 hours and 72 km downstream in 12 hours. The speed of the boat in still water is:

Given:

Distance upstream (D_u) = 80 km

Time upstream (T_u) = 16 hours

Distance downstream (D_d) = 72 km

Time downstream (T_d) = 12 hours

Formula used:

Speed upstream (S_u) = D_u / T_u

Speed downstream (S_d) = D_d / T_d

Speed of boat in still water (B) = (S_u + S_d) / 2

Calculations:

S_u = 80 / 16

⇒ S_u = 5 km/hour

S_d = 72 / 12

⇒ S_d = 6 km/hour

B = (S_u + S_d) / 2

⇒ B = (5 + 6) / 2

⇒ B = 11 / 2

⇒ B = 5.5 km/hour

∴ The correct answer is option 4.