Acar starts from rest and accelerates along a straight line path in one minute. it finally attains a velocity of 40 meters/second. what is the car's average acceleration?

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.