A cylindrical water tank of radius 3.5 $m$ and height 5 $m$ stands open at the top. A flat circular lid is to be made for it from sheet costing ₹150 per $m^2$. Find the cost of the lid in rupees. Take π = 22/7.

The lid is just the circular top: area = π$r^2=(22/7)\times 12.25=38.5m^2$, so the cost = $38.5\times 150$ = ₹5775. The height 5 $m$ is not needed anywhere. (Making a base as well as a lid doubles it to ₹11550; using the diameter 7 $m$ as the radius gives $154{\text{m}}^2$ and ₹23100; charging the 22 $m$ rim at ₹150 gives ₹3300, which prices a length at an area rate.)