A cylindrical tin has internal radius 6 cm and internal height 16 cm. Find the length of the longest thin straight rod that can be laid inside it, in cm.

The longest rod runs from a point on the rim at the top to the diametrically opposite point on the base, so it is the diagonal of the axial rectangle whose sides are the DIAMETER, 12 cm, and the height, 16 cm: √$(12^2+16^2)=$ √400 = 20 cm. (Using the radius 6 cm instead of the diameter gives √292 ≈ 17.09 cm; 22 cm adds 6 and 16 instead of using Pythagoras; 16 cm is just the height, which any rod standing upright would match.)