Correct answer: 45 cm
The center of mass is calculated using the formula: \( x_{cm} = \frac{\sum m_i x_i}{\sum m_i} \). Substituting the values: \( x_{cm} = \frac{(300 \times 0) + (500 \times 40) + (400 \times 70)}{300 + 500 + 400} = \frac{0 + 20000 + 28000}{1200} = 45 \, \text{cm} \).