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Boats and Streams - General Aptitude

Each of the questions given below consists of a question followed by three statements. You have to study the question and the statements and decide which of the statement(s) is/are necessary to answer the question.
Q1:

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 Any two of the three

ANS:D - Any two of the three

From I and II, speed of boat in still water = 1 (12 + 4) km/hr = 8 km/hr.
2
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).