Turner's treadmill runs with a velocity of -1.2 m/s and speeds up at regular intervals during a half-hour workout. after 25 min, the treadmill has a velocity of -6.5 m/s. what is the average acceleration of the treadmill during this period?

0.000353 m/
In this case, during 25 minutes of acceleration the treadmill velocity changed from -1.2m/s into -6.5m/s. Since the velocity and time unit is different, you need to convert the time from minutes to seconds too. To find the acceleration, the calculation would be 

acceleration = final velocity - start velocity / time
acceleration= (-6.5m/s) - (-1.2m/s) / (25 minute x 60 second/min)= (-5.3m/s )/ 15000 sec
accerelation = 3.53 x m/ = 0.000353 m/