Answer:
She will have 104 boxes.
Explanation:
Thing of it like this:
100% of 20 is 20, so 400% would be 80. Then the last 20% could be answered like this:
divide 20 by 10. Since it is 20%, multiply by 2. You get 4. Add the sums to get 104. The reason you divide 20 by 10 is because its easier and since 2x10 is 20, then 20/10 is reasonable.