Physics - one-dimensional motion with constant acceleration?

A glider on an air track carries a flag of length l through a stationary photogate that measures the time interval [box]td during which the flag blocks a beam of infrared light ping across the gate. The ratio vd = l/ [box]td is the average velocity of the glider over this part of its motion. Suppose the glider moves with constant acceleration. (a) Argue for or the against idea that vd is equal to the instantaneous velocity of the glider when it is halfway through the photogate in terms of distance. (b) Argue for or against the idea that vd is equal to the instantaneous velocity of the glider when it is halfway through the photogate in terms of time.