Since the event thet it rains and is windy is included in bothe events it rains tommorow and it is windy tommorow if we add the probabilites that it rains tommorow and that it is windy tomorrow we have counted the event that it both rains and is windy tommorow twice, so we need to subtract it once to get the required answer.

Its a case of the inclusion-exclusion principal (yes the answer is 0.6).

CB