We want to compute the probability P(A or B) because we could either solve it following path A, path B, or doing both paths.
P(A) = 1/3
P(B) = 2/5
P(A and B) = P(A)*P(B) assuming A,B are independent
P(A and B) = (1/3)*(2/5)
P(A and B) = 2/15
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P(A or B) = P(A) + P(B) - P(A and B)
P(A or B) = 1/3 + 2/5 - 2/15
P(A or B) = 5/15 + 6/15 - 2/15
P(A or B) = (5 + 6 - 2)/15
P(A or B) = 9/15
We could reduce this to 3/5, but it appears your teacher has chosen not to.
