<PRE><B>Question 982790</B>


let speed of car be x

time taken = 460/x

speed increased by 15 km/h

new speed = x+15

new time = 460/(x+15)

saves 1.5 hours =3/2

460/x - 460/(x+15) = 3/2

(460(x+15)-460x)/x(x+15) = 3/2

460x+6900-460x = (x^2+15x)*3/2

6900*2/3 =(x^2+15x) 

4600 = x^2+15x

x^2+15x-4600=0

solve the quadratic equation 
x= speed =60.7 km/h

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