To find out how many ice cream cones Margaret had at noon, we multiply $ \frac{2}{3} $ by 72. Since 72 is divisible by 3, we can simplify $ \frac{2}{3}\cdot 72 $ by re-writing it as $ 2\cdot \frac{72}{3} $, which equals $ 2\cdot 24=48 $. To find how many ice cream cones she had at the end of the day, we multiply $ \frac{2}{3} $ by 48 to find $$ \frac{2}{3}\cdot 48 = \frac{2\cdot 48}{3} = 2\cdot \frac{48}{3} = 2\cdot 16 = \boxed{32} $$