Question

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

• A. 12
• B. 9
• C. 8
• D. 6

Answer 1

Answer:

B. 9

Given:

Speed of Current = 3 kmph

Distance = 12 km

Calculation:

Let the speed of boat in still water be x kmph, then

$\frac{12}{x+3}+\frac{12}{x-3}=3$

$12[\frac{x-3+x+3}{x^2-3^3}]=3$

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

$8x=x^2-9$

$x^2-8x-9=0$

$x^2-9x+x-9=0$

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

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

$x=9, x=-1$

Speed of Boat can't be in negative

So Speed of Boat in Still Water is 9 kmph.