SOLUTION: A snowstorm causes a bus driver to decrease the usual average rate along a 60-mile route by 15 miles per hour. As a result, the bus takes two hours longer than usual to complete th

Question 412926: A snowstorm causes a bus driver to decrease the usual average rate along a 60-mile route by 15 miles per hour. As a result, the bus takes two hours longer than usual to complete the route. At what average rate does the bus usually cover the 60-mile route?
My book says that the Answer is 30mph. I suspect the problem has to do with Time = Distance / Rate, but am unsure how to set up the problem properly. Please help!
Answer by mananth(16946) (Show Source): You can put this solution on YOUR website!
let usual speed be x mph
time with decreased speed - time with usual speed = 2
60/(x-15) - 60/x =2
LCD = x(x-15)
multiply by the LCD
60x-60(x-15)= 2x(x-15)
60x-60x+900=2x^2-30x
2x^2-30x-900=0
/2
x^2-15x-450=0
x^2-30x+15x-450=0
x(x-30)+15(x-30)=0
(x-30)(x+15)=0
x= 30 mph the original speed
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