Question

the boat goes 20 km upstream in 5hours and 35km downstream in 7hours find the speed of boat in still water,speed of the current

Answer 1

Let speed of boat be
x km/ hr
and speed of water current be y km/hr

Distance = speed * time

• speed of boat upstream = (x - y) km/hr because stream is opposing the speed of boat.


• speed of boat downstream = (x + y) km/hr because stream is adding it's speed to the speed of boat.
According to the question :

I condition :
20 = 5 * (x -y) -------(1)
II condition :
35 = 7 * (x+y) -------(2)

Adding equation (1) and (2)

=>55 = (5x-5y) + (7x+7y)
=>55 = 5x-5y+7x+7y
=>55 = 12x + 2y
=>y= (55 - 12x)/2 --------(3)

put the value of y in equation (1)

20=5{x-[(55-12x)/2)]}
=> 4= x - 55/2 + 12x/2
=> 4+55/2 = x + 6x
=> 63/2= 7x
=> x = 9/2
put in equation (3)

y = (55-12*9/2)/2
=> y = (55-54)/2
=> y = 1/2

Therefore speed of boat is 4.5 km/hr
and speed of stream will be 0.5 km/hr.


Hope it helps