The lower triangle with sides 15 ft , 20 ft and c is a right triangle. The hypotenuse measures c.
Use the Pythagorean theorem to find c.
15^2 + 20^2 = c^2
225 + 400 = c^2
c^2 = 625
c = 25
Side c measures 25 ft.
The two triangles are similar, so the lengths of corresponding sides are proportional.
120/20 = d/15
6 = d/15
d = 90
Length d is 90 ft.
