Answer:15ft
Explanation:
First look at the triangles made by the ladder,you can find that they are right angled triangle.
so their one side is right angle(90 degrees),and one of the other angle is equal as given in the question.
because two angles in both triangle are equal both triangles can be considered similar,
we know that sides of similar triangle are proportional so we can write,
(also let the length of short ladder be x),
40/10=60/x
4x=60
:.x=15ft