Answer :
If the ratio of the sides in both triangles is the same then it will be true to say that the two triangles are similar.
This means, the ratio of their corresponding sides is equal
In this case,
40 / 30 = d/90
4/3 = d/90
4* 90 = 3*d
360 = 3d
360/3 = d
120 = d
d = 120 ft
Answer
120 ft.