help with random selection

[kkohn]

If a class has 18 juniors, 10 seniors. 6 of the seniors are female and 12 of the juniors are males. If a student is selected at random, what is the probability of selecting?

- A junior or a female

- a senior of a female

-junior or a senior

I am curious about what formula I could use to most easily answer this question and hopefully use these 3 examples to get me started

 

[galactus]

#1: Let J=junior and F=female.

P(J or F)=P(J)+P(F)-P(J and F)

#2: S=senior and F=female

P(S or F)=P(S)+P(F)-P(S and F)

#3: They can not be a junior and senior at the same time, so P(S or J)=P(S)+P(J)