Quantitative Aptitude Ques 587
Question: The speed of the current is 5 km/h. A motorboat goes 10 km upstream and back again to the starting point in 50 min. The speed of motorboat in still water is
Options:
A) 20 km/h
B) 26 km/h
C) 28 km/h
D) 25 km/h
Show Answer
Answer:
Correct Answer: D
Solution:
- Let speed of boat be x km/h.
$ \therefore $ Upstream speed of boat $ =(x-5),km/h $ Downstream speed of boat $ =(x+5) $ $ \frac{10}{x-5}+\frac{10}{x+5}=\frac{50}{60} $
$ \Rightarrow $ $ 10( \frac{x+5+x-5}{x^{2}-5^{2}} )=\frac{50}{60} $
$ \Rightarrow $ $ 6\times 2\times 2x=x^{2}-25 $
$ \Rightarrow $ $ x^{2}-25-24x=0 $
$ \Rightarrow $ $ x^{2}+25x+x-25=0 $
$ \Rightarrow $ $ (x-25)(x+1)=0 $
$ \Rightarrow $ $ x=25,km/h $
BETA
