You should always remember that a cylinder consists of circles that are all over each other to form the height of the cylinder.
So the volume of the cylinder= the area of the surface circle* the height.
[tex]V=540=\pi r^2*h= \pi r^2*15 [/tex]
Then,
[tex] r^2= \frac{540}{15 \pi } [/tex]
So, [tex]r= \sqrt{\frac{540}{15 \pi }} [/tex]
radius is approximately 3.4cm
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Hope this helps.