i did not understand how the coordinates of the center of mass of a small square come to a : a/4 , a/4

Not able to understand

What will be the centre of mass in case of a trolley bag having wheels??

How to solve this question?

In Question Where Solid sphere And Hollow Sphere are compare In This Outer Radius Is Given Same So Why We Get That Moment Of Inertia Of Solid Is Greater Than Hollow