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In the formula P(EB) = P(E) * P(B/E)
P(EB) stands for P of event E and event B happening together, when they're dependent.
P(E) stands for P of event E happening.
p(B/E) stands for P of event B happening on condition that event E has happened.
THis way the formula makes sense:
50% * 40% = 20%.
So, i have
p(B/E) = 40%!
I don't think it's good to use conditional formula here. Simply get 4C1/5C2, and you've got the answer! Remember that the guy has ALREADY chosen E, so we can disregard E as an event. It has already happened.