Hello! To since this, all we have to do is divide fractions. To divide fractions, keep the first fraction the same, change division into multiplication, and flip the second fraction over. Then, multiply straight across. 2 1/4 as an improper fraction is 9/4. 9/4 * 4/1 is 36/4 or 9 when simplified. It will take 9 kids to fill the prism.