Quantitative Aptitude Ques 1299
Question: In a stream running at 5 km/h, a motorboat goes 1 km upstream and back again to the starting point in 35 min. Find the speed of the motorboat in still water.
Options:
A) 12 km/h
B) 5 km/h
C) 7 km/h
D) 14 km/h
Show Answer
Answer:
Correct Answer: C
Solution:
- Let the speed of the motorboat in still water be x km/h. Downstream speed $ =(x+5)km/h $ Upstream speed $ =(x-5)km/h $
$ \therefore $ $ \frac{1}{x+5}+\frac{1}{x-5}=\frac{35}{60} $
$ \Rightarrow $ $ \frac{2x}{x^{2}-25}=\frac{7}{12} $
$ \Rightarrow $ $ 7(x^{2}-25)=24x $
$ \Rightarrow $ $ 7x^{2}-24x-175=0 $
$ \Rightarrow $ $ 7x(x-7)+24(x-7)=0 $
$ \Rightarrow $ $ (x-7)(7x+24)=0 $ $ x=7 $ Thus, speed of the motorboat in still water $ =7km/h $
BETA
