Question 519968
let speed be x

t=d/r

time with original speed - time with increased speed = 1
90/x-90/(x+3)=1
LCD=x(x+3)
multiply by LCD
90(x+3)-90x=x(x+3)
90x+270-90x=x^2+3x

x^2+3x-270=0
x^2+18x-15x-270=0
x(x+18)-15(x+18)=0
(x+18)(x-15)=0
x=15 ignoring negative value

speed = 15 mph

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