- 4.5 km/hr
- 4 km/h
- 9 km/hr
- 5.6 km/hr

Option 1 : 4.5 km/hr

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If ‘x’ and ‘y’ are speed of the person in still water and speed of the stream respectively.

Speed of the person downstream = x + y

Speed of the person up stream = x – y

Given,

The person can row one km upstream in ten minutes

∴ Distance covered = 1 km

Time required = 10 min = 10/60 hr = 1/6 hr

∴ Distance covered = Speed is upstream × Time required

⇒ 1 = (x – y) × (1/6)

⇒ x – y = 6 ----(1)

The person can row one km downstream in four minutes

∴ Distance covered = 1 km

Time required = 4 min = 4/60 hr = 1/15 hr

∴ Distance covered = Speed in downstream × Time required

⇒ 1 = (x + y) × (1/15)

⇒ x + y = 15 ----(2)

(2) – (1)

x + y – x + y = 15 – 6

⇒ 2y = 9

⇒ y = 9/2 = 4.5 km/hr
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