I've spent a long time looking at this and cannot do it. My teacher explained it, but went too fast. Could somebody explain to me how to do this calculation, and why it is done the way it is.

The bones of an adult human skeleton have a mass of approximately 9kg. Of this, 1kg is calcium.

The calcium in bones can be assumed to be present as calcium phosphate. A phosphate ion has the formula PO4 (3-)

a.) What is the formula of calcium phosphate

- Ca3PO4(2)

b.) Estimate the percentage by mass of calcium phosphate in an adult human skeleton.

I'm not sure how to do this. I did 1/9 times 100, and this is wrong.

Could somebody explain how to do this please.