A-man-can-row-a-boat-at-4km-hr-in-still-water-He-rows-the-boat-2km-upstream-and-2km-back-to-his-starting-place-in-2hours-How-fast-is-the-stream-moving-please-help-

Question Number 12699 by chux last updated on 29/Apr/17

A man can row a boat at 4 km / hr

in still water . He rows the boat 2 km

upstream and 2 km back to his

starting place in 2 hours . How fast

is the stream moving ?

please help

Answered by mrW1 last updated on 29/Apr/17

x = s p e e d o f w a t e r

\frac{2}{4 - x} + \frac{2}{4 + x} = 2

\frac{8}{16 - x^{2}} = 1

x^{2} = 8

x = 2 \sqrt{2} \approx 2.83 (k m / h)

Commented by chux last updated on 29/Apr/17

thanks sir .

pls i dont understand the concept

of the speed of the upper and lower

stream .

Commented by mrW1 last updated on 30/Apr/17

w h e n t h e m a n r o w s a g a i n s t t h e s t r e a m,

i f t h e w a t e r w e r e s t i l l, t h e c o u l d r o w

4 k m i n o n e h o u r . i n t h i s o n e h o u r t h e

w a t e r b r i n g s i h m x k m b a c k, i . e . h i s

s p e e d r e l a t i v e l y t o t h e c o a s t i s (4 - x) k m / h r .

w h e n h e r o w s i n t h e d i r e c t i o n o f t h e

s t e a m, h i s s p e e d r e l a t i v e l y t o t h e

c o a s t i s (4 + x) k m / h r . s i n c e h e r o w s

2 k m u p p e r s t e a m a n d 2 k m b a c k i n

t o t a l l y 2 h o u r s, w e h a v e

\frac{2}{4 - x} + \frac{2}{4 + x} = 2

Commented by chux last updated on 30/Apr/17

thanks \dots . i really appreciate this .

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