Given P(A fails) = 0.2 P(B fails alone) = 0.15 P(A' $\cap$ B') = 0.15.
P (B fails alone) = 0.15 = P (B') - P (A' $\cap$ B') $\rightarrow$ 0.15 = p (B') - 0.15 $\rightarrow$ P (B') = 0.3.
(i) P (A fails | B has failed) $= \large\frac{P (A' \cap B')}{P(B')} = \large\frac{0;15}{0.30} $$= 0.5$
(ii) P (A fails alone) = P (A fails) - P (both A and B fail) = P (A') - P (A' $\cap$ \ B') = 0.2 - 0.15 = 0.05