RRB RPF RPSF SI and CONS Model Papers 7 with solutions

Question : 67 of 120

Marks: +1, -0

A boat goes 12 km down- stream and comes back to the starting point in 3 hours. If the speed of the current is 3 km/hr, then the speed (in km/ hr) of the boat in still water is

Validate

Solution:

(2) Tricky Approach
If the speed of boat in still water be x kmph, then

{12}/{x+3}+ {12}/{x-3}=3

⇒ 12 ( {x-3+x+3}/{(x+3)(x-3)} )=3

⇒ 4 × 2 x=x^{2}-9

⇒ x^{2}-8 x-9=0

⇒ x^{2}-9 x+x-9=0

⇒ x(x-9)+1(x-9)=0

⇒(x-9)(x+1)=0

⇒ x=9

because

x ≠-1

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