Is the average velocit over a specific timeinterval always half of the instantaneous velocity at the end of the interval.

I said False, because if the curveis not linear this would not be true.

Would this be a a sufficent answeror am I missing anything.

The answer is false, unless the velocity was zero at the beginning of the interval and the acceleration was constant. This is often not the case. Why should the velocity always be zero at the beginning of an arbitary time interval? It can be anything.