An obtuse triangle has measurements of 100°,60° and 20°. If the longest side of this triangle is 20ft what is the length of the shortest side?

The longest side of a triangle is always opposite the largest angle of a triangle.

If the smallest angle of the triangle is 20 degrees, then the shortest side is opposite to it. Using this information, we can form the equality

20/100 (longest leg/biggest angle) = x/20 (shortest leg/smallest angle)

We can cross multiply this to get 400 = 100x. From here it is a simple "find x" and the shortest side is 4 feet.