We will solve as follows:
First: We determine the number of multiples of 5 between 1 and 100:
So, there are 20 multiples of 5 between 1 and 100 [Inclusive].
Second: We determine the probability of not chosing a multiple of 5:
So, the probabilty is 4/5 that the number chosen at random will not be a multiple of 5.