The circumference of the bas of the cylinder is 18π units. The cylinder is 30 units tall.
Which of the following is the closest to the volume of the cylinder?
A 706 cubic units
B 1,414 cubic units
C 7,630 cubic units
D 21,195 cubic units

The volume of a cylinder is    (pi) (radius)² (height).

We know the height, but we don’t know the radius.
Is there a way we can find it?
We know the circumference of the base.   Maybe that’s a clue.
Can you find the radius of a circle when you know the circumference?

             Circumference of a circle =  (pi) x (diameter)
                                    18 pi units = (pi) x (diameter)

Divide both sides by pi :  18 units = diameter

Divide both sides by 2 :    9 units = radius      

Now we know the radius and the height of the cylinder,
so we can calculate the volume.

radius = 9
height = 30
we’ll use  3.14  for pi.

Volume = (pi) (radius)² (height) =

             (3.14)    (9)²      (30) =

             (3.14)   (81)      (30) =   7,630.2 cubic units

Is there a choice close to that?  I’ll go up and look.   

Yes !  Choice ’C’ is  7,630 cubic units.