A man goes downstream with a boat to some destination and

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A man goes downstream with a boat to some destination and returns upstream to his original place in 5 hours. If the speed of the boat in still water and the stream are 10 km/hr and 4 km/ hr respectively, the distance of the destination from the starting place is

1. 16 km
2. 18 km
3. 21 km
4. 25 km

Correct Option: C

Let the distance of the destination from the starting point be d km.
Rate downstream = (10 + 4) kmph = 14 kmph
Rate upstream = (10 – 4) kmph = 6 kmph
According to the question,

d	+	d	= 5
14		6

⇒	3d + 7d	= 5
	42

⇒ 10d = 42 × 5

⇒ d =	42 × 5	= 21 km
	10

We can find the required answer with the help of given formula :
Here, p = 10, q = 4, t = 5

d =	t(p² - q²)
	2p

d =	5(10² - 4²)
	2 × 10

d =	5(100 - 16)
	2 × 10

d =	84	= 21
	4