A circular pond of radius 14 m is surrounded by a path 3.5 m wide running all around it. What is the cost of gravelling the path at ₹40 per square metre? (Use π = 22/7.)

The outer radius is 14 + 3.5 = 17.5 m, so the path is a ring of area π$(17.5^2-14^2)=(22/7)\times 110.25=346.5$ sq m, costing $346.5\times 40$ = ₹13860. The trap ₹38500 gravels the whole outer circle of 962.5 sq m without removing the pond; ₹1540 squares the width, π $x3.5^2=38.5$ sq m, instead of taking the difference of the squares; ₹12320 uses the pond's circumference $x$ width = $88\times 3.5=308$ sq m, which ignores that the ring widens as it goes outwards.