The height of the telephone tower is 120 feet.
The first step to solve this problem is to figure out what fraction of the height of the actual object the shadow is. We can find this by dividing the length of the shadow by the actual height.
4/6 = 2/3, meaning that the shadow is 2/3 of the actual height
Since we know that the shadow is 2/3 of the height, we can multiply by the reciprocal, 3/2, to find the actual height if we know the length of the shadow.
80 · 3/2 = 120, so the height of the telephone tower is 120 feet.