It is very important to note down all the details that are given in the question. Based on these details the answer can be derived.
Initial velocity of the car (V1) = 0 meters/second
Final velocity of the car (V2) = 40 meters/second
Initial time (T1) = 0 Second
Final time (T2) = 60 seconds (In this case we have converted 1 minute into 60 seconds for the ease of calculation.
Then
Average acceleration = (V2 - V1)/ (T2 - T1) m/s^2
= (40 - 0)/(60 - 0) m/s^2
= 40/60 m/s^2
= 2/3 m/s^2
= 0.67 m/s^2
So the average acceleration of the car is 0.67 meters/second square.