ANS:D - 50 cm

To find the depth of the sewerage, we can use trigonometry with the given angle. Let's consider the sewer water surface as a circular segment, with the center of the sewer being the center of the circle. The angle subtended by the surface of the sewer water with partial flow is given as 120°. The formula to find the depth (h) of the circular segment given the angle (θ) is: h=r (1−cos(θ/2​)) Where:

• ℎh = depth of the circular segment (depth of the sewerage in this case)
• r = radius of the circle (which is the radius of the sewer)
• θ = angle subtended by the circular segment (given as 120°)