Quantitative Aptitude Ques 1125

Question: A boat goes 12 km downstream and comes back to the starting point in 3 h. If the speed of current is 3 km/h, then the speed (in km/h) of the boat in still water is

Options:

A) 12

B) 9

C) 8

D) 6

Show Answer

Answer:

Correct Answer: B

Solution:

  • If the speed of boat in still water be $ xkm/h. $ Then, speed in downstream $ =(x+3)km/h $ Speed in upstream $ =(x-3)km/h $ According to the question, $ \frac{12}{x+3}+\frac{12}{x-3}=3 $

$ \Rightarrow $ $ \frac{12x-36-12x+36}{x^{2}-9}=3 $

$ \Rightarrow $ $ 3x^{2}-27=24x $

$ \Rightarrow $ $ x^{2}-8x-9=0 $

$ \Rightarrow $ $ (x-9)(x+1)=0 $

$ \Rightarrow $ $ x\ne 1 $

$ \therefore $ $ x=9km/h $