1. 

What is the speed of the boat in still water ?

I. The speed downstream is 12 kmph.

II. The speed upstream is 4 kmph.

III. In a to and fro journey between two points, the average speed of the boat was 6 kmph.
A. I and II only
B. All I, II and III
C. III, and either I or II
D. Any two of the three
E. None of these

Answer: Option D

Explanation:

From I and II

speed of boat in still water = 1 (12 + 4) km/hr
2

= 8 km/hr.

From II and III, we get:

Using average speed = 2xy , we get: 2 x 4 x y = 6
x + y 4 + y

8y = 24 + 6y

y = 12.

Required speed = 1 (12 + 4) km/hr = 8 km/hr.
2

Similarly, I and III also give the answer.

Correct answer is (D).