SOLUTION: On a journey of 300km, the train driver calculates that if he reduces his average speed by 10km/h he would take 25 mins longer. Find his average speed

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Question 544480: On a journey of 300km, the train driver calculates that if he reduces his average speed by 10km/h he would take 25 mins longer. Find his average speed
Answer by mananth(16946) About Me  (Show Source):
You can put this solution on YOUR website!
Original speed x
Changed speed x - 10

Distance = same 300 miles
original time – time with decreased speed =0.42
t=d/t
300/(x-10)-300/x= 5/12
300x-300(x-10)= 5*x(x-10)/12
300x-300x+3000=(5x^2-50x)/12
multiply by 12
36000=5x^2-50x
5x^2-50x-36000=0
/5
x*2-10x-7200=0
x^2-90x+80x-7200=0
x(x-90)+80(x-90)=0
(x-90)(x+80=0
x= 90 mph the average speed.