a
The distance of the center of mass of the system of three masses from the origin \(O\) is

\({X_{CM}} = \frac{{{m_1}{x_1} + {m_2}{x_2} + {m_3}{x_3}}}{{{m_1} + {m_2} + {m_3}}}\)
\(= \frac{{300 \times 0 + 500 \times 40 + 400 \times 70}}{{300 + 500 + 400}}\)
\(= \frac{{500 \times 40 + 400 \times 70 400\left[ {50 + 70} \right]}}{{1200 1200}}\)

\(= \frac{{50 + 70}}{3} \cdot  = \frac{{120}}{3} \cdot  = 40\,cm\)