Originally Posted bylingyai

For c) you need to observe that:

p(a AND b)+p(a AND (NOT b))=p(a)=1-p(NOT a),

two of these terms have already been established in parts a) and b).

Plug these in and you will find:

0.4+p(a AND (NOT b))=0.65,

which will give the given solution.

RonL