Salut/Hello!
Answer: 20 ft
Explanation:
We can imagine the ladder, wall and ground as a triangle.
(And because we don't know if our triangle has a 90 degrees angle it means it's an ordinary triangle.)
We know that a triangle that has an angle with 60 degrees means all of its angles have 60 degrees because 180 : 3 angles = 60 degrees which means the triangle is equilateral.
Definition of the equilateral triangle: The triangle that has the 3 side with the exact same length.
From the equilateral triangle we know all sides are equal. If the ladder is 20 ft that means the distance between the building and the base of the ladder (which is actually the base of our triangle) is 20 ft.
I hope it was useful! :]