There's 100 J for growth from a total of 3000J ingested. To find out what percentage of J are used for growth, you need to figure out how much 100 of 3000 is. To do this, you divide 100/3000=0.0333. To turn it into a percent, you have to times it by 100 again, so 0.0333 x 100 = 3.33.
