A boat can move 35 km upstream and the same distance downstream in a total time of 8 hours. If the speed of the boat in still water is 9 km/h done the speed (in km/h)of the stream is:
Show answer & explanation
Correct answer: C
Explanation
The correct answer is option 3 i.e. 1.5.
Let the speed of the stream be x km/h
Upstream speed = 9 − x km/h
Downstream speed = 9 + x km/h
Total time = (35/(9 − x)) + (35/(9 + x)) = 8
⇒ 35(9 + x) + 35(9 − x) = 8(81 − x2)
⇒ 35(9 + x + 9 − x) = 8(81 − x2)
⇒ 35 × 18 = 648 − 8x2
⇒ 630 = 648 − 8x2
⇒ 8x2 = 648 − 630 = 18
⇒ x2 = 18/8 = 2.25
⇒ x = = 1.5