If it is normally distributed, then 50% of drivers are traveling at speeds of 48mph or greater. The cutoff point is 0.6 standard deviations from the mean . So, use that to find the percent of people driving greater than 45 mph. In other words, find the z-score that is associated to that number of standard deviations from the mean.
For (b), again, this is based on numbers of standard deviations. If you want the top 10%, find how many standard deviations that is equal to, multiply that number by 5, and add it to 48.