Assume that you are the Marketing Executive at ABC Ltd$.$ You know that sales targets are normally distributed with a mean of $3,900$ units and a standard deviation of $160$ units. What is the probability that randomly selected sales personnel will have a sales target that is:

a$.$ Less than $3,500$ units

b$.$ Greater than $4,200$ units

c$.$ Between $3,800$ and $4,100$ units