1) A police officer with a speed gun was distracted and failed to check the speed of a grey minivan that passed by. 2) The officer heard the minivan's engine at a frequency of 459Hz after it passed. Typically, minivan engines operate at 500Hz. 3) Using the Doppler effect formula and information about sound speed and the speed limit, the assistant determines that the minivan was traveling at 110km/hr, exceeding the 90km/hr speed limit. Therefore, the minivan was speeding.