Question

Concrete can be purchased by the cubic yard. How much will it cost to pour a circular slab 18 ft in diameter by 3 in. for a patio if the concrete costs $40.00 per cubic yard?
(1 cubic yard = 27 cubic feet)

Answer 1

So hmmm check the picture below

now

$\bf V=\pi 9^2\cdot \cfrac{1}{4}\implies V=\cfrac{81\pi }{4} \\\\\\ \textit{there are }27ft^3\ in\ 1yd^3\textit{ thus, let's convert them to }yd^3 \\\\\\ \cfrac{\frac{81\pi }{4}}{27}\iff \cfrac{\frac{81\pi }{4}}{\frac{27}{1}}\implies \cfrac{81\pi }{4}\cdot \cfrac{1}{27}\implies \boxed{\cfrac{3\pi }{4}}$

now, that's how many yard³ are there for that slab, now, how much is the cost?

well, each yd³ costs $40, thus   $\bf 40\cdot \cfrac{3\pi }{4}$