A car accelerates at a constant rate from zero to 28.7 m/s in 10 seconds and then slows to 17.5 m/s in 5 seconds. What is its average acceleration to the nearest tenth of a m/s2 during the 15 seconds?

Answer 1

car changes from 0m/s to 17.5m/s in 15 sec.

Looks like an average acceleration of 17.5m/s / 15s = 1.1666 m/s^2

It doesn't matter how long it did what. The average acceleration only takes into account change in speed and time elapsed.