By increasing the speed of a car by 10 kilometre per hour the time of a journey of 72 km is reduced by 36 minutes find the original speed of the car
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let the original speed be xkmph
time= 72/x
new time= 72/x - 36/60
=72/x- 3/5
=(360-3x)/5x
new speed= x+10
hence
72 = (x+10)× (360-3x)/5x
360x= 360x-3x^2 + 3600- 30x
3(x^2+10x-1200)=0
(x+40)(x-30)=0
so x= -40
or x= 30
hence original speed is 30 kmph
time= 72/x
new time= 72/x - 36/60
=72/x- 3/5
=(360-3x)/5x
new speed= x+10
hence
72 = (x+10)× (360-3x)/5x
360x= 360x-3x^2 + 3600- 30x
3(x^2+10x-1200)=0
(x+40)(x-30)=0
so x= -40
or x= 30
hence original speed is 30 kmph
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