Please. This is a basic word riddle. You are supposed to think that it means divided by 3. It doesn't, it actually means divide by a third, or multiply by 3. 15 ÷ 3 = 5. 15 ÷ 1/3 = 45. You add 15, then you do divide by 3, and you get three piles of 20. Done. End of story.