SOLUTION: On some highways, a car can legally travel 20 km/hr faster than a truck. Travelling at maximum legal speeds, a car can travel 120 kms in 18 minutes less than a truck. What ar

The time equation is


     -  =  =   of an hour.


To solve it, multiply both sides by x*(x-20).  You will get


    1200x - 1200(x-20) = 3x*(x-20)

        24000 = 3x*(x-20)

         8000 = x*(x-20)      (*)


At this point, you just can guess the solution  x= 100.


ANSWER.  the maximum speed is  100 km/h for cars and 80 km/h for trucks.


CHECK.    -  =  =  =  =  =  of an hour = 18 minutes.  ! Correct !