Answer: 15 feet
Explanation:
Given: Quadrilateral ABCD is similar to quadrilateral EFGH.
If the two shortest sides of quadrilateral EFGH are 6 feet long and 12 feet long.
Let x be the shortest side of quadrilateral ABCD .
The measurement of sides of quadrilateral ABCD arranged in order :-
60 feet , 40 feet, 30 feet and x feet.
We know that the corresponding sides of two similar polygons are proportional.
⇒ The two shortest sides of quadrilateral ABCD is proportional to the two proportional sides of quadrilateral EFGH.
⇒

Hence, the length of the 4th sides of quadrilateral ABCD = 15 feet.