Question: how do you work out the volume of a circle?
Answer: First, work out the area of one of the circular ends of the cylinder. The area of a circle is πr2 (π × radius × radius). π (pi) is approximately 3.14. ... 
3.14 x 2.5 x 2.5 = 19.63cm2 The volume is the area of an end multiplied by the length, and is therefore:
19.63cm2 x 20cm = 392.70cm3