To solve this we have to apply trigonometric functions:
The length of the ladder is the hypotenuse of the triangle (x), the distance between the base of the tower and the night is the adjacent side (15). So, we have to apply:
Cos 60°= adjacente/hypotenuse:
Cos 60°=15/x
Solving for x:
x= 15/cos 60°
x = 15/0.5
x=30 feet