Question

plz clarify my doubt
why we need to put (-) instead of (+)
ANS IT

Answer 1

Let me solve it in another way.

= > Let the speed the stream be 'x'.

= > Speed of the boat upstream = (5 - x)km/hr.

= > Speed of the boat downstream = (5 + x)km/hr.

Now,

Given it takes 1 hour more to row the boat upstream than to return downstream.

= > (5.25)/(5 - x) = (5.25)/(5 + x) + 1

= > (5.25)/(5 - x) - (5.25)/(5 + x) = 1

= > (5.25)[(1/(5 - x) - 1/(5 + x)] = 1

= > (5.25)[(5 + x - 5 + x)/(5 + x)(5 - x)] = 1

= > (5.25)[2x/25 - x^2] = 1

= > (10.5x) = 25 - x^2

= > 10.5x - 25 + x^2 = 0

= > x^2 + 10.5x - 25 = 0

= > x^2 + 12.5x - 2x - 25 = 0

= > x(x + 12.5) - 2(x + 12.5) = 0

= > (x - 2)(x + 12.5) = 0

= > x = 2 (or) x = -12.5(Neglect negative value).

Speed of stream = 2km/hr.

Hope this helps!