i think that you could go through every number, but first, you should eliminate some of the obvious numbers. this way, you'll be able to go through the numbers you are looking through and count those.
i started by eliminating all of the even numbers, as those will all share a factor with 28, since 28 is divisible by 2.
then i eliminated 21 and 28.
now you are left with -
11, 13, 15, 17, 19, 23, 25, 27, and 29.
so, there are 9 integers.
hope this helped!