Probability - I'm confused, please help!

A traffic officer has a concealed radar unit that she uses to measure the speed of traffic crossing a bridge. She finds that the mean speed is 84km/h and the standard deviation is 5km/h.

a) What probability distribution is most likely to model the speed of the traffic crossing the bridge? Explain why you made your choice and give any parameter(s) for your distribution.

b) If the speed limit on the bridge is 90km/h, find out how many out of 200 cars she would expect to find to be breaking the limit.