For travel from Washington DC to Baltimore by car

You have to give either the distance between Washigton and Baltimore, or the time it took them to drive back and forth in order to calculate the average speed.

d = distance = rate * time

d is the same for each leg of the trip....out and back

60 * t1   =   40 * t2       divide both sides by 60

t1 = 2/3 t2

60 * t1   +    40 * t2   = a (t1 + t2)                      sub in    t1 = 2/3 t2

60 (2/3 t2)   +   40 t2   = a (2/3 t2 + t2)              divide both sides by   t2

40 + 40 = 5/3 a

a = 48 m/hr