A quick and short "naive" solution

In 20 minutes, a truck is   = 20 km ahead the car.


Moving faster than the truck, the car has approaching speed of 80-60 = 20 km/h.


Therefore, it is clear that the car will overtake the truck in  = 1 hour.    ANSWER

        A slow Algebra solution

Let t be the time after the car started.

So, to the catching moment, the car moves t hours.


The truck started 20 minutes earlier, so the truck moved (t+1/3) hours till the car catches up the truck.


They cover the same distance, so we write this distance equation

    80*t = 60*(t+1/3)


Left side is the distance covered by the car; right side is the distance covered by the truck.


To solve, multiply both sides of the equation by 3. You will get

    240t = 60*(3t+1)


Simplify and find t

    240t = 180t + 60

    240t - 180t = 60

        60t     = 60

          t     = 60/60 = 1  hour.


So, the car overtakes the truck in 1 hour after the car' start.


You get the same answer.