4. The dean has observed for several years and found that the probability distribution of the salary of the alumni's first job after graduation is normal, with a mean of $50k per year and a standard deviation of $5k. Please show all work, as I want to learn this skill.

(a)What's the probability that the alumni get a salary of $45k or more in their first jobs?

(b)What's the chance that the alumni get a salary between $35k and $55k?

(c)What is the salary range that 95% of the alumni should be belonged to?