Question

The speed of a boat downstream is 2.5 times its speed upstream. If the total time taken by the boat for going 15 km downstream and the distance upstream is \(4\frac{2}{3}\) hours, then what is the speed (in km/h) of the boat downstream?
1. \(11\frac{1}{2}\)
2. \(11\frac{1}{4}\)
3. \(10\frac{5}{8}\)
4. 10

Answer 1

Correct Answer - Option 2 : \(11\frac{1}{4}\)

Given:

The speed of the boat downstream is 2.5 or (5/2) times of upstream. They take 4(2/3) hours to complete the journey in both the direction by covering a distance of 15 on each side.

Formula Used:

Time = Distance/Speed

Calculation:

Let the speed of the boat while going downstream and upstream be 5x and 2x. Now apply the Formula mentioned.

⇒ 15/5x + 15/2x = 4(2/3) = 14/3

⇒ 15(1/5x + 1/2x) = 14/3

⇒ 15(7x/10x²) = 14/3 or x = 9/4

Now the spedd of the boat downstream is 5x will be

⇒ 5×(9/4) = 45/4 = 11(1/4)

∴ The spedd of the boat in downstream is 11(1/4)