What is the diagonal of 16x12?

The length of the diagonal is found using the Pythagorean theorem

It is the square root of (162 +122 )=the square root of 400=20

Note, if you know about 3,4,5 triangles, then you could multiple each number by 4 and have a
12,16,20 triangle. The rectangle in question if made of two of those.