Imagine that the probability of answering this question correctly is only p = .4, and on tests in which it is used, only 8 people attempt it. On what proportion of such tests will fewer than 3 people answer the question correctly?
This is the same as 1 or 2 people answering correctly.
The probability of both/all events occurring is determined by multiplying the probabilities of the individual events.
For 1, .4(.6)^7 = ?
For 2. (.4)^2(.6)^6 = ?
Either-or probabilities are found by adding the individual probabilities.