Question 1143512
<br>
I see the "symmetrical cross of thickness 12 cm" as being made up of five squares, each with side length 12cm.  The area of the cross is 5*12^2 = 720 square cm.<br>
The area of the cardboard is pi(20^2) square cm.<br>
The percent of waste is {{{(area of cardboard-area of cross)/area of cardboard}}}, converted to a percent.<br>
You can do the calculations.