The probability that student A will fail is .5, the probability that student B will fail is .2, and the probability that both will fail is .1. So..

P(A)=.5 P(B)=.2 P(AB)=.1

What is the probability that at least one student will fail?

Exactly one student will fail?

I did .5+.2-.1= .6 for P of at least one. I'm not sure if this is right or if it is for exactly one student instead. Any help please?