Question

Ratio of the speed of boat A in still water and speed of boat B in still water is 5:4 and the ratio of the speed of current and downstream speed of boat A is 1:6. If boat A covers 80 km upstream in 4 hours, then find downstream speed of boat B?
A. 20 kmph
B. 25 kmph
C. 24 kmph
D. 30 kmph
E. 28 kmph

Answer 1

Upstream speed of boat, $A=80km=\frac{80}{4}=20kmph$

Downstream speed of boat A=6x

Speed of stream =x

Speed of boat A-x=20

Speed of boat A=6x-x=5 x

$\begin{array}{l}5 x-x=20 \\x=5 \mathrm{kmph}\end{array}$  

$Speed of boat \mathrm{A}=5^{*} 5=25kmph\\ \mathrm\\ Speed of boat \mathrm{B}=25 * 4 / 5=20 \mathrm{kmph}\\ Downstream Speed of boat B=20+5=25 \mathrm{kmph}$

Answer 2

As per data given in the question,

We have to find the value of downstream speed.

Let the speed of boat = x kmph

Speed of stream = y Kmph

So, up-speed stream will be = (x-y) Kmph

Down-stream speed will be = (x+y) Kmph

So, as per question,

Upstream speed of boat, $A=80 \mathrm{~km}=\frac{80}{4}=20 \mathrm{kmph}$

Downstream speed of boat will be $A=6 x$

Speed of stream $=x$

So,

Speed of boat $A-x=20$

Speed of boat $A=6 x-x=5 x$

Hence, from the question,

$\begin{array}{l}5 x-x=20 \\x=5 \:\mathrm{kmph}\end{array}$

Speed of boat $\mathrm{A}=5{\times} 5=25 \mathrm{kmph}$

Speed of boat $\mathrm{B}=25 \times \frac{4}{5}=20 \:\mathrm{kmph}$

Downstream Speed of boat $B=20+5=25\: \mathrm{kmph}$