Explanation

(b) Co-ordinate of CM is given by
$X_{CM} = \frac{m_{1}x_{1} + m_{2}x_{2}}{m_{1} + m_{2}}$
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Taking parts $A$ and $B$ as two bodies of same system
$m_{1} = l \times b \times \sigma = 8 \times 2 \times \sigma = 16\sigma$
$m_{2} = l \times b \times \sigma = 6 \times 2 \times \sigma = 12\sigma$
Choosing $O$ as origin,
$x_{1} = 1\ m,\ x_{2} = 2 + 3 = 5\ m$
$\therefore$ $X_{CM} = \frac{16\sigma \times 1 + 12\sigma \times 5}{16\sigma + 12\sigma} = \frac{19}{7}$
$= 2.7\ m\ from\ O$