posted by Mingma Sherpa .
A number cube is rolled three times. What is the probability of rolling a 5 at least once?
Getting "a 5 at least once" = one, two or three 5s.
Once = 1/6(5/6)(5/6) = ?
Twice = 1/6(1/6)(5/6) = ?
Three times = 1/6^3 = ?
Either-or probabilities are found by adding individual probabilities.