The balanced chemical reaction is:
FePO4 + Na2SO4 = Fe2(SO4)3 + Na3PO4
We are given the amount of FePO4 is given to be used in this reaction. This will be the starting point for the calculations.
20.0 g FePO4 ( 1 mol FePO4/ 150.82 g FePO4 ) ( 1 mol Fe2(SO4)3 / 1 mol FePO4) ( 399.88 g Fe2(SO4)3/ 1 mol Fe2(SO4)3) = 53.0274 g Fe2(SO4)3