The 420 freshmen at an engineering college take either calculus or discrete mathematics (but not both). Further, both courses are offered providing either an introduction to a CAS (computer algebra system) or using such a system extensively throughout the course. The results in Table 3.6 summarize how the 420 freshmen are distributed.
Table 3.6

(a) If Sandrine is taking calculus, what is the probability her class is only being introduced to the use of a CAS?
(b) Derek's class is making extensive use of the CAS. What is t|ie probability Derek is taking discrete mathematics?

CAS CAS (Introduction) (Extensive Calculus Discrete Mathematics 170 80 Coverage) 120 50