If a ball is thrown into the air with avelocity of 80ft/s, its height in feet after t seconds is given byy= 80t-16t^2. find its averge velocity from the time its thrown to the time it reaches its maximan height.

here is what i did

first i took the dervative of the function and equalled it to zero. i got t= 2.5.

then i calculated the average velocity i got 32 where it should be 40. i guess that i made a mistake while calculating the average velocity. can someone help me from there.

thanks in advance!!