How to solve a probability problem?

Question

Probability theory

The worker made n parts. Let the event Ai consist in the fact that the i-th part manufactured by him has a defect. Record an event that: at least two parts are free of defects

Answer 1

As Rsa97 already mentioned , it's easier to describe an event that is opposite to the given one, and then "hang" negation on the resulting event. Event opposite to the one specified: all parts are defective or only one part is defective.
Those. the desired event B looks like this:
or the same, but including the space of elementary events:

Answer 2

1 - (probability that all parts are defective) - (probability that all but the first part are defective) - (probability that all but the second part are defective) - ... - (probability that all , except for the nth, the parts are defective)