The correct answer is answer choice C, three. In the formula Ca3 (P O4)2, there are 3 atoms of calcium (Ca), 2 atoms of phosphorus (P), and 8 atoms of oxygen (O). You can tell this by looking at the subscripts (or numbers, in this case, since you can't type subscripts). The subscript applies to the number directly beside it. Furthermore, if you have parentheses with a number beside it, just distribute. In this problem, we have (P O4)2.
(P O4) 2 Given
P2 O8 Distribute
We now have 2 atoms of phosphorus and 8 atoms of oxygen.
This problem has decided to be nice and just ask for calcium, so we don't have to distribute. All you have to do for this problem is look at the subscript beside Ca, which is 3. Therefore, the correct answer is C.
