∵ [(9 + 6)/(9 - 6)] x v = 12

Correct Option: A

∵ [(9 + 6)/(9 - 6)] x v = 12
∴ v = 2.4 km/hr

Let x be the speed of the boat and y be the speed of the current.
Speed upstream = (x - y) km/h
Speed downstream = (x + y) km/h

Correct Option: D

Let x be the speed of the boat and y be the speed of the current.
Speed upstream = (x - y) km/h
Speed downstream = (x + y) km/h
According to the question,
20 /(x - y) + 20/(x + y) = 25/36
In this equation, there are two variables but only one equation. So, the value of x cannot be determined.

Given, speed of boat = 10 km/h
Let speed of flow of river = x km/h
∴ Upstream speed of boat = (10 - x) km/h
and downstream speed of boat = = (10 + x) km/h

According to question.
91/(10 - x) + 91/(10 + x) = 20

Correct Option: A

Given, speed of boat = 10 km/h
Let speed of flow of river = x km/h
∴ Upstream speed of boat = (10 - x) km/h
and downstream speed of boat = = (10 + x) km/h

According to question.
91/(10 - x) + 91/(10 + x) = 20
⇒ 91(10 + x + 10 - x) / (10 - x) (10 + x) = 20
⇒ 91(20) / 100 - x² = 20
⇒ 91 = 100 - x²
⇒ x² = 9
∴ x = 3

Let the distance between M and N and the speed of current be d km and x km/hr respectively.
According to the question = {d/(4 + x) + d/(4 - x)} = 3

Correct Option: D

Let the distance between M and N and the speed of current be d km and x km/hr respectively.
According to the question = {d/(4 + x) + d/(4 - x)} = 3

In the above equation we have only one equations but two variables. Hence cannot be determined (Data inadequate).

Speed upstream = (6 - 1.5) km/hr = 4.5 km/hr
Speed downstream = (6 + 1.5) km/hr = 7.5 km/hr

Correct Option: C

Speed upstream = (6 - 1.5) km/hr = 4.5 km/hr
Speed downstream = (6 + 1.5) km/hr = 7.5 km/hr
Total time taken = (22.5/4.5 + 22.5/7.5) hrs
= (5 + 3) hrs.
= 8 hrs.