how many two digit positive integers are divisible by 5 but not three

I think that you need to recount geno.

The two digit positive numbers divisible by 5 are:  15, 25, 30, ..., 90, 95

(there are 17 of them).

Of them, those that are divisible by 3 are 15, 30, 45, 60, 75, and 90

(there are 6 of them).

That means, that there are 11 remaining ...