Estimate the volume in metric units and compute the acual volume of the item:
Oil in a barrel that has a height of 1 m and a diameter of 0.5m
The answer in the book: 0.20mcubed
$\displaystyle V = \pi r^2h $
$\displaystyle r = \frac {d}{2} = \frac{0.5\ m}{2} = 0.25\ m.$
$\displaystyle \Rightarrow V = \pi \times (0.25)^2 \times 1$
$\displaystyle \Rightarrow V = 0.20\ m^3 $ (correct to $\displaystyle 2$ sig. fig.)
I hope that helps.
ILoveMaths07.