According to Avogadro's Number there are 6.022 * 10²³ atoms in 1 mol of atoms. So:
1 mol of atoms of P = 6.022 * 10²³ atoms of P
We are asked to find the number of moles that we have in 3.20 * 10²⁵ atoms of phosphorus.
number of moles = 3.20 * 10²⁵ atoms of P * 1 mol of P /(6.022 * 10²³ atoms of P)
number of moles of P = 53.1 moles
Then the answer to our problem is 53.1 moles.