What is the perimeter of a square wraith an area of 900 square ft?

The lcm is never less than the larger number.  An easy way to generate the
pairs you're looking for is to make the larger one a multiple of the smaller
one.  Then the larger one is always their lowest common multiple.

2  and  20 . . . lcm is  20 .

3  and  39 . . . lcm is  39 .

5  and  45 . . . lcm is  45 .