<PRE><B>Question 792404</B>
Doreen rate --------x mph----------------------56 miles
sue rate ------------x+15 mph------------------88 miles

t=d/r

Sue time - Doreen time = 4 hours


56/x - 88/(x+15) =4

multiply by x(x+15)

56(x+15)-88x=4x(x+15)

56x+840-88x=4x^2+60x

-32x+840=4x^2+60x

4x^2+60x+32x-840=0

4x^2+92x-840=0

/4
x^2+23x-210=0

x^2+30x-7x-210=0

x(x+30)-7(x+30)=0

(x+30)(x-7)=0

x=-30 OR x=7

ignore negative

x= 7 speed of Doreen 
7+15 = 22 mph speed of Sue



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