A prism is rectangular shaped,. a pyramid is triangular shaped.
The ratio would be 1 to 3
The formula for the pyramid is V = 1/3Bh
A century is 100 years, and a millenium is 1000 years.
A centrury is 100/1000 part of millenium, or simplifying 1/10 or 0.10 part of millenium (or 10%).
