Suppose mass per unit are a of the disc = m
Mass of the disc, M = π (d/2)2 m = πmd2/4
Mass of portion removed,
M' = π(d/12)2 m = πmd2/144
Let centre O of the disc be taken as the origin. Suppose masses M and M' are concentrated at O and O' respectively. Fig,
As the portion is removed, taking M' as negative, CM of the remaining portion is at a distance 'x' from 'O' where
Negative sign indicates CM is to the left of origin O.