This is a proportion problem.
Let's make her Length x and her height 1/3 x.
1/3 x = 20 inches
x = 6 2/3 inches
Molly's height is 6 2/3 inches.
Check it.
6 2/3 x 3 = ? (change the mixed number to a fraction)
20/3 x 3 = ? (multiply)
60/3 = ? (divide)
20 inches. Yes, it checks.