Boats and Streams - Important Points
Boats and Streams
Boats and streams problems are asked in quantitative aptitude and are considered under time and distance. Let’s first understand the concept of boats and streams.
Let B = Speed of the boat in still water and
S = Speed of the stream (river)
Downstream and Upstream
Downstream motion means moving with the stream and upstream motion means moving against the stream.
Downstream Speed (D) = B + S
Upstream Speed (U) = B – S
So, B = (D + U)/2 and S = (D – U)/2
Example 1: A boat can travel with a speed of 15 km/hr in still water. If the speed of the stream is 5 km/hr, find the time taken by the boat to go 80 km downstream.
Solution: Speed downstream = (15 + 5) km/hr = 20 km/hr.
Time taken to travel 80 km downstream = 80/20= 4 hrs
Example 2: A man travels downstream for 5 hours and again upstream for 5 hours. Yet it is at a distance of 10 kms from the starting point. What is the speed of stream?
Solution: Using the formula, 5(B + S) – 5(B – S) = 10
10 S = 10 => S = 1 km/hr
Example 3: A man takes 8 hrs to row 75 km downstream and come back in 8 hrs. If the speed of boat in still water is 20 km/hr, find the speed of stream.
Solution: Time taken in Downstream + Time taken in upstream = 8 hrs
75/(B + S) + 75/(B – S) = 8
75/(20 + S) + 75/(20 – S) = 8
S = 5 km/hr