Need to do some unit conversions here. They give speed in mi/h but want time in seconds and give distance in feet. Easiest thing to do would be to convert the speed to ft/s:
15 mi/h x 5280 ft/mi x 1 h / 3600s = 22ft/s
Now solve for time:
Speed = distance / time
22ft/s = 3300ft / t
t = 3300ft / 22ft/s
t = 150s