Original system (with 4 particles) Let coordinates of the particle as ‌A=(0,a) ‌B=(a,a) ‌C=(a,0) ‌D=(0,0) The centre of mass of system X‌com ‌‌=‌
mxA+mxB+mxC+mxD
4m
Y‌com ‌‌=‌
myA+myB+myC+myD
4m
Solving, ‌Xcom=‌
0+a+a+0
4
=‌
a
2
‌Ycom=‌
a+a+0+0
4
=‌
a
2
The centre of mass of original system at (‌
a
2
,‌
a
2
) New centre of mass, when one particle (B) is removed,