Dobby enjoyed apparating $($disappearing from one location and appearing in another location after a short amount of time$)$ for fun to surprise his fellow magic $1$ His goal was to apparate to different displacements and see how fast he could get from the start to $1600m$ reapparating every $400m.$ Dobby's running totals ev $400m$ were $15s,38s,58s,$ and $81s$ respectively for the $400m,800m,1200m,$ and $1600m.$

What was Dobby's average acceleration for the total mile?