Question

In a River, Direction of the current flow is changes consecutive of 1 hour (if 1^st hour current direction is from point A to B than in next 1 hour direction of current is B to A) till 12 P.M. and after 12 P.M. water be still, Boat continuously streaming from 10 A.M to 3 P.M. in speed of current 3 km/hr. and downstream speed of boat is 15 km/hr. than find the distance covered by boat.

1. 50 km
2. 60 km
3. 70 km
4. 58 km
5. None of these

Answer 1

Correct Answer - Option 2 : 60 km

Given:  

Current speed = 3 km/hr.

Downstream speed of boat is 15 km/hr.

Boat continuously streaming from 10 A.M to 3 P.M.

Direction of the current flow is changes consecutive of 1 hour (if 1^st hour current direction is from point A to B than in next 1 hour direction of current is B to A) till 12 P.M. and after 12 P.M. water be still

Concept:

Speed of boat in still water = speed of boat in downstream – speed of current flow

When current direction alternate after 1 hr. consecutive then automatically effect of current In 2 hr. (or of even number) is cancelled.

Calculation:

Current speed = 3 km/hr.

Speed of boat in still water = speed of downstream – speed of current flow

⇒ Speed of boat in still water = 15 – 3

⇒ Speed of boat in still water = 12 km/hr.

Now 10 am to 3 pm is 5hr. of period (10 am to 12 pm which is even interval in number and there effect of current cancel.

Which means all we calculate with speed of still water.

Distance cover by boat = 12 × 5

Distance cover by boat = 60 km

∴ Distance cover by boat is 60 km.