∵ Speed of current = 8 - 36/6 = 2 km/hr
⇒ Rate downstream = (8 + 2) = 10 km/hr

Correct Option: D

∵ Speed of current = 8 - 36/6 = 2 km/hr
⇒ Rate downstream = (8 + 2) = 10 km/hr
∴ Required distance = (8 + 2) x 10 = 100 km

Speed of water current =( V_d - V_u)/2

Correct Option: C

Speed of water current =( V_d - V_u)/2
= (21 - 15) /2
= 3 km/h

S_d = 15 km/h
S_u = 9 km/h
Speed of man= (S_d + S_u)/2

Correct Option: D

S_d = 15 km/h
S_u = 9 km/h
Speed of man= (S_d + S_u)/2
∴ Speed of man = (15 + 9)/ 2 = 12 km/h

Downstream speed of boat = Speed of boat in still water + Speed of stream
Required time = Distance/Speed downstream

Correct Option: A

Downstream speed of boat = Speed of boat in still water + Speed of stream
= 24 + 8 = 32 km/h
∴ Required time = Distance/Speed downstream
= 128/32 = 4 km/h

Velocity of the current = (speed downstream - Speed upstream)/2

Correct Option: D

Speed upstream = Distance/Time = 60/10 = 6 km/h
Speed upstream = Distance/Time = 36/10 = 3.6 km/h
∴ Velocity of the current = (speed downstream - Speed upstream)/2
= (6 - 3.2)/2 = 1.2 km/h