A singly reinforced concrete beam of 25 cm width and 70 cm effective depth is provided with 18.75 cm2 steel. If the modular ratio (m) is 15, the depth of the neutral axis, is A. 20 cm B. 25 cm C. 30 cm D. 35 cm

The correct answer is C. 30 cm.

The depth of the neutral axis (c) can be calculated using the following formula:

$c = \frac{bd}{As + bd/m}$

where:

• b = beam width = 25 cm
• d = effective depth = 70 cm
• As = area of steel = 18.75 cm2
• m = modular ratio = 15

Substituting these values into the formula, we get:

$c = \frac{(25 \text{ cm})(70 \text{ cm})}{(18.75 \text{ cm}^2) + (25 \text{ cm})(70 \text{ cm})/15}$

$c = 30 \text{ cm}$

The other options are incorrect because they do not correspond to the correct value of the depth of the neutral axis.