Step-by-step explanation
If the height of an object h varies directly with the length of its shadow l then both quantities are related by an expression like this one:

Where k is a number. We are told that a 6 ft tall person has a 15 ft long shadow. This means that for h=6 we have l=15. Then we can construct an equation for k:

We can divide both sides by 15:

So the relation between height and the length of the shadow is:

Then if the tree is 20 ft tall we get:

We can divide both sides by 0.4:

Answer
Then the answer is 50 ft.