What is the probability that a two digit-number chosen at random
will be a multiple of 3 and not of 5?
There are 90 two-digit numbers.
One-third of them are mutliples of 3: . multiples of 3.
But every fifth one of them is a multiple of 5: . multiples of 15.
Hence, there are: . multiples of 3 which are not multiples of 5.