Question

The base of a lampshade is in the

Answer 1

This whole problem is only about a circle.  Let's not get confused by the rest of the lampshade.

The circumference of any circle is    (pi) times (the diameter) .

For this circle, it's      (pi) times (13 cm).

They told us what to use for (pi), so it's    (3.14) x (13 cm) = 40.82 cm

and that's all there is to it.