How many atoms of phosphorus are in 4.20 mol of copper(ii) phosphate?

The formula for copper(II) phosphate is: Cu3(PO4)2
Thus, for every mol of Cu3(PO4)2, there is 2 mol of phosphate atoms.

2 x 4.40 = 8.80 mol.

Using Avagadro's number, we can now calculate the number of phosphate atoms:

N = n x N(A)
(N=number of atoms, n=amount in mole, N(A)=6.02x10^23 - Avagadro's number)

Therefore:
N = 8.80 x (6.02x10^23)
= 5.29 x 10^24

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have a good day