not sure which average this is but one of the averges is 12.5 m/s^2... it's been a while since physics so I'm a little rusty
btw, to find this average you have to divide 60 by 4 to get 15m/s^2.
next you divide 60 by 6 to get 10m/s^2.
then you add 15 and 10 to get 25.
finally, you divide 25 to get 12.5m/ s^2.