Isn't it suppose to be: 18 to 20 is 2 20 to 30 is 10 30 to 33 is 3 2 + 10 + 3 = 15 ? Also, how does she know it's 18, plus an additional 18? Why not

Isn't it suppose to be: 18 to 20 is 2 20 to 30 is 10 30 to 33 is 3 2 + 10 + 3 = 15 ? Also, how does she know it's 18, plus an additional 18? Why not 18 + 20? What happens if it's 33 minus 21? 21 + 21 is 42... Just curious - but at least she's got her own system that works!