a) the probability of none of them hitting

= the complement of at least one of them hitting

=P( ( A or B or C)' )

=1-P( A or B or C)

=1- [P(A)+P(B)+P(C)-P(AB)-P(AC)-P(BC)+P(ABC)]

b) the required P = P(AB'C' or A'BC' or A'B'C)

the above 3 cases are mutually exclusive..

so the ans = adding the probability of each case

c) similar approach as (b)