math
posted by Javi .
Estimate the cost of the concrete requred to make a concrete sidewalk shaped as follows.
(The sidewalk covers 1/4 of the outer perimeter of a circle  from 3:00 to 6:00. The radius from the center of the circle to the outside perimeter is 20 feet. The radius to the inner circle/left side of sidewalk is 15 feet. So the width of the sidewalk is approx. 5 feet wide, covering 1/4 of circle.)
The depth of the concrete is 4 inches. The concrete costs $105/cubic yard.
thank you.

area of walk is pi/2 (20^2  15^2) = pi/2 * 175 = 274.889 ft^2
volume of concrete is 274.889 ft^2 * 1/3 ft * yd^3/27ft^3 = 3.39 yd^3
cost is thus 105 $/yd^3 * 3.39yd^3 = $356.34