A runner starts slowing down as she gets tired. At 10 seconds, she 8m/a, and slows down to 2m/a at 13 seconds. What is her acceleration.

Magnitude of acceleration = (change in speed) / (time for the change).

Change in speed = (end speed) - (start speed)

= (2 m/a) - (8 m/a) = - 6 m/a.

Time for the change = (end time) - (start time)

= (13 sec) - (10 sec) = 3 sec.

Magnitude of acceleration = (-6 m/a) / (3 sec)

= - 2 m/a per second.

(That's the best I can do with the ambiguous units.

What unit of speed is 'm/a'? Is that "miles per afternoon"?)