Two distinct natural numbers from 1 to 9 are picked at random. What is the probability that their product has 1 in its unit place?

Two dice are thrown. What is the probability that difference of numbers on them is 2 or 3 ?

What is the mean of the numbers 1, 2, 3, ... 10 with frequencies ⁹C₀, ⁹C₁, ⁹C₂ ..., ⁹C₉, respectively?

The probability that a person recovers from a disease is 0.8. What is the probability that exactly 2 persons out of 5 will recover from the disease?