Assuming that the 40 and 30 refer to sides of a rectangle and not to any of the infinite number of other possible shapes, the answer is 50.
A square with a 30-inch diagonal measurement has sides of 21.21 inches in length.
The diagonal length is about 20.59
Sqrt(282 + 402) = a whisker under 48 ft 10 in
67'1"
The square has a diagonal measurement of: 1.13 km
42
32.311
A square with a 30-inch diagonal measurement has sides of 21.21 inches in length.
Using Pythagoras the diagonal is 20 times square root of 2
46.648 ft
That would be 34 feet.
The diagonal measurement of an 8 ft square is: 11.31 feet.
The diagonal length is about 20.59
Since the rectangle has right angles, you can use Pythagoras' Theorem in this case.
The diagonal is 20.
The diagonal measurement of an 18 x 18 square is: 25.46 units.
The diagonal measurement for 36 feet by 36 feet is 50.91 feet.