Answer :

(i) Let the speed of Ritu in still water and the speed of stream be x km/h and y km/h respectively.
While rowing upstream, Ritu's speed slows down and the speed will be her speed minus speed of stream and while rowing downstream her speed will increase and will be equal to sum of her speed and speed of stream. Therefore,

The speed of Ritu while rowing

Upstream = (x - y) km/h

Downstream = (x + y) km/h


According to the question:

Ritu can row downstream 20 km in 2 hours, and
distance = speed x time

⇒ 2 (x+y) = 20

⇒ x+y = 10............(1)


also, 
Ritu can row upstream 4 km in 2 hours

⇒ 2 (x - y) = 4
⇒ x-y = 2 ............(2)


Adding equation (1) and (2),

we obtain

⇒ x + y + x - y = 10 + 2

⇒ 2 x = 12
⇒ x = 6


Putting this in equation (1),

6 + y = 10

we obtain y = 4


Hence, Ritu's speed in still water is 6 km/h and the speed of the current is 4 km/h.
(ii)Let the number of days taken by a woman and a man be x and y respectively.

Therefore, work done by a woman in 1 day =


and work done by a man in 1 day =


According to the question,

2 women and 5 men take 4 days to complete the work 
i.e. they take 4 days to complete one work

⇒ 


⇒ 


Also, 
3 women and 6 men take 3 days to complete the work i.e. 
they take 3 days to complete one work

⇒ 


⇒ 


Putting in these equations,


we obtain

⇒ 8p + 20q = 1


and


⇒ 9p + 18q = 1


By cross-multiplication, we obtain







x = 18, y = 36


Hence, number of days taken by a woman, x = 18
Number of days taken by a man, y = 36


(iii) Let the speed of train and bus be u km/h and v km/h respectively.

According to the given information,

It takes her 4 hours if she travels 60 km by bus and rest ( i.e. 240 km) by train
As

we have, 


and also,  If she travels 100 km by train and the remaining by bus, she takes 10 minutes longer i.e. 4 hours and 10 minutes
Also, 
 1 hour = 60 minutes

and

we have, 


Putting in these equations, we obtain


60p+240q = 4 ......(3)


100p+200q =


600p+1200q = 25 ....(4)


Multiplying equation (3) by 10, we obtain


600p+2400q = 40 ....(5)


Subtracting equation (4) from (5), we obtain


1200q = 15


..... (6)


Substituting in equation (3), we obtain


60p+3 = 4


60p = 1




u = 60km/h and v = 80km/h


Hence, speed of train = 60 km/h


Speed of bus = 80 km/h

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