Question:

Three masses are placed on the x-axis: 300g at origin, 500g at x=40cm and 400g at x=70cm. The distance of the centre of mass from the origin is?

45cm

40cm

50cm

30cm

Let COM be at point C.
∴Xcm = (m1X1 + m2X2 + m3X3) / (m1 + m2 + m3)
Xcm = (300 × 0 + 500 × 40 + 400 × 70) / 1200
= 48000 / 1200
Xcm = 40cm