Question 412926
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