To solve this problem you have to set up a proportion. To form one side of the proportion you use "is over of" meaning what number represented by is, in this case 18, is placed over the number represented by of, 3000. 
Since you are finding the percent of something, youre x value is placed above "100" because percent values are most commonly out of 100%. 
So now your proportion will look like To solve the proportion you cross multiply and divide. So you multiply 100 and 18 since they are crossed from each other, then divide 1800 by 3000. The answer is .6. 18 is .6% of 3000.
So now your proportion will look like