Question

A number is chosen at random from the set of numbers (21,22,23,.......34,35) what is the probability that it is

Answer 1

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 $35-21+1 = 15$ 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 $\frac{3}{15}=\frac{1}{5}$

There are three prime numbers, namely 23,29 and 31. So, the probability of choosing one of them is $\frac{3}{15}=\frac{1}{5}$

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 $\frac{0}{15} = 0$

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 $\frac{15}{15} = 1$