Correct Answer - Option 4 : 120 Km
Given:
The speed of the boat in the still water = 15 km/hr
The speed of the stream = 5 km/hr
Let the Distance travelled by boat in upstream and downstream be d
Total time is taken by the boat to cover both upstream and downstream = 9 hrs
Formula Used:
Distance = Speed × Time
Upstream boat speed (x-y) = speed of the boat in still water – the speed of the stream
Downstream boat speed (x+y) = speed of the boat in still water + speed of the stream
The formula for this method:
t = \([\frac{d}{{(x + y)}} + \frac{d}{{(x - y)}}]\)
d is the distance
x is the speed of the boat in still water.
Y is the speed of the stream
t is the time taken
Calculation:
Substituting the values in the formula
Speed of the boat in upstream (x - y) = 15 – 5 ⇒10 Km/hr
Speed of the boat in downstream (x + y) = 15 + 5 ⇒ 20 km/hr
Distance travelled by upstream and downstream
= \([\frac{d}{{10}} + \frac{d}{{20}}]\) ⇒ 9
3D = 9 × 20
D ⇒\([\frac{{180}}{3}]\) = 60 Km
∴ Total distance both upstream and downstream = 60 + 60 ⇒ 120 km