You are given:  initial velocity   = 40 km/h;
                ending  velocity   = 80 km/h;

                travel distance   d = 30 km.


To find the time, start from these two kinematic equations


    d =  + ,      (1)

    a = ,            (2)


where " a " is the acceleration.  Substitute (2) into (1), eliminating "a".  You will get


    d =  =  =  =  = .


Thus you started from the formal kinematic formula (1)  and ended with intuitively understandable formula


    d = .


Now you only need substitute the given data to get simple equation


    30 = 

    30 = 60t

     t =  =   of an hour = 30 minutes.    ANSWER