How many numbers from 1 to 18,000, inclusive, are divisible by 20 or 3 or both?
$\displaystyle 18000 = 900\cdot20 $
So all the multiples of $\displaystyle 20 $ are $\displaystyle \{1\cdot20,2\cdot20,3\cdot20,\cdots,900\cdot20\} $
Hopefully you can see how many elements are in that set.
The case with $\displaystyle 3 $ follows the same way.