ANS:C - 30 cm

To determine the depth of the neutral axis of a reinforced concrete beam, we use the formula: x=​m⋅σs/σc ​​⋅d Where:

• x = Depth of the neutral axis
• m = Modular ratio (given as 15)
• σs​ = Stress in steel (given as 250 kg/cm²)
• σc​ = Stress in concrete (given as 62.5 kg/cm²)
• d = Effective depth of the beam (given as 70 cm)

Given:

• m=15
• σs​=250 kg/cm²
• σc​=62.5 kg/cm²
• d=70 cm

Substituting these values into the formula, we have: x=15×250/62.5 ​×70 x=3750/62.5​×70 x=60×70 x=4200 cm So the calculated depth of the neutral axis is 4200 cm. However, the unit of the answer is in centimeters, which is not practical for a beam. This discrepancy suggests that there might be an error in the units provided or in the calculation process. If we assume that the value of 4200 is meant to be in millimeters rather than centimeters, then the depth of the neutral axis would be 4200 mm. In this case, the depth of the neutral axis would be 4200/10​=420 cm, which is not among the provided options. Given the options provided, it seems that none of them match the calculated value. Therefore, it's possible that there might be a mistake in either the options or the given parameters.