Question

5
If Ritu can row downstream 20 km in 2
hours, and upstream 4 km in 2 hours,
then her speed of rowing in still water
and the speed of current respectively
is :
(1 Point)
7 km / hr ,10 km/hr
6 km / hr, 4 km/hr
15 km/hr, 21 km/hr
none of the above​

Answer 1

Step-by-step explanation:

$\mathfrak{\huge{\orange{\underline{\underline{Answer :}}}}}$

Let speed of ritu in still water be 'x'

Let speed of current be 'y'

Upstream

While ritu is moving upstream , the speed of the current opposes the speed of ritu

So,

Speed = x - y

Given that she takes 2hrs to row upstream.

So, t = 2

distance = 4km (given)

We know that,

speed = $\bf\frac{distance}{time}$

x - y = 4/2

x - y = 2 -------------eqn(1)

━━━━━━━━━━━━━━━━━━━━

Downstream

While ritu is moving upstream , the speed of the current supports the speed of ritu

So,

speed = x + y

Given that she takes 2hrs to row downstream

So, t = 2

distance = 20km(given)

speed = distance /time

x + y = 20/2

x + y = 10 ----------------eqn(2)

━━━━━━━━━━━━━━━━━━━━

Adding equation 1 and 2(elimination method)

x - y = 2

x + y = 10

━━━━━

2x. = 12

x = 12/2

x = 6

Therefore,speed of ritu in still water is '6 kmph'

Now , put x = 6 in eqn 2

x + y = 10

6 + y = 10

y = 10 - 6

y = 4

Therefore,speed of current is '4 kmph'

Therefore, the correct answer is option(B)