Question

A boat takes 6 hours to travel 36 km downstream and 24 km upstream same boat takes 9 hours to go 48 km downstream and 40 km upstream. Find the speed of stream and speed of boat in still water.



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Answer 1

   Total Time =
Upstream distance/upstream speed + Downstream dist/downstream speed  
Let speed of boat=x
Let speed of stream=y
Upstream speed = speed of boat - speed of stream = x - y 
Down stream speed = speed of boat + speed of stream = x + y
Given 36/x+y + 24/x-y = 6 hours...... (i)
48/x+y + 40/x-y = 9 hours.......(ii)
Multiply eq.(i) by '5' and eq.(ii) by '3'
180/x+y + 120/x-y = 30.....(iii)
144/x+y + 120/x-y = 27.....(iv)
Subtract above equations:
36/x+y = 3
x+y = 12.....(a)
(a) in (iv)
144/12 + 120/x-y = 27
12 + 120/x-y = 27
120/x-y = 15
x-y = 8
x+y = 12
Adding both:
2x = 20
x=10
10+y = 12
y=2
Speed of boat in still water = 10 kmph
Speed of stream = 2 kmph

Answer 2

https://m.imgur.com/a/COyav6k