area of right circular cylinder

Question

A tunnel was constructed for the metro-railway and any cross-section of the inside curved is a semi-circle with a diameter 8m. If the cost of painting the inside curved surface of the tunnel is $1,000 at the rate of $3 per meter square. Find the length of tunnel.?

Answer 1

total cost of painting of tunnel is $1000 but cost to paint 1 m2 is 3 so area is $1000/3/m2 is 333.3m2 therefore is pie *r2*h=333.3 implies that 333.3*7/22*16=6.628 m is length of tunnel