Acar traveling at 25 m/s speed up to 40 m/s over a period of 15 seconds. the average acceleration of the car is? why?

A car with an initial velocity of 25 m/s speeds up to 40 m/s over a period of 15 seconds. The average acceleration of the car is 1 m/sec/sec. It changed 15 m/sec in 15 seconds.

answer; i believe the correct answer is (replication is not conservative);