Question

Twice the speed of a boat downstream is equal to thrice the speed upstream. The ratio of its speed in still water to its speed in current is 

(a) 1 : 5 

(b) 1 : 3 

(c) 5 : 1 

(d) 2 : 3

Answer 1

(c) 5 : 1

Let the speed downstream be u km/hr and speed upstream be v km/hr. 

Given, 2u = 3v ⇒ u = \(\frac{3v}{2}\)

Required ratio = \(\frac12(u+v):\frac12(u-v)\)

= \(\bigg(\frac{3v}{2}+v\bigg):\bigg(\frac{3v}{2}-v\bigg)\)

= \(\frac{5v}{2}:\frac{v}2=5:1.\)