In cases like this, the probability of a certain event is given by the ratio between the positive outcomes, and all positive outcomes. There are [tex] 35-21+1 = 15 [/tex] numbers in your set. The positive outcomes depend on the request:
There are three multples of four, namely 24,28 and 32. So, the probability of choosing one of them is [tex] \frac{3}{15}=\frac{1}{5} [/tex]
There are three prime numbers, namely 23,29 and 31. So, the probability of choosing one of them is [tex] \frac{3}{15}=\frac{1}{5} [/tex]
The number 18 doesn't belong to the set, so you can't possibly choose it. In other words, there are zero positive outcomes, so the ratio becomes [tex] \frac{0}{15} = 0 [/tex]
All of the numbers in the set are whole number, so you are certain that you will chose a whole number. In other words, all 15 possible outcomes are positive outcomes, so the ratio becomes [tex] \frac{15}{15} = 1 [/tex]
