A boat goes 20 km downstream in one hour and the same distance

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A boat goes 20 km downstream in one hour and the same distance upstream in two hours. The speed of the boat in still water is

1. 15 km/hr.
2. 10 km/hr.
3. 5 km/hr.
4. 7.5 km/hr.

Correct Option: A

Let the speed of boat in still water be p kmph and the rate of stream be q kmph.
Distance = 20 km
∴ Downstream rate= (p + q) kmph and upstream rate = (p – q) kmph.

Now,	20	= 1
	p + q

⇒ p + q = 20 ......(i)

and	20	= 2
	p - q

⇒ p – q = 10 ...(ii)
From (i) and (ii) we have p = 15 kmph.
We can find the required answer with the help of given formula :
Here, y = 20, t₁ = 1, t₂ = 2

Speed of Boat =	y		1	+	1
	2		t₁		t₂

Speed of Boat =	20		1	+	1
	2		1		2

Speed of Boat = 15 km/hr