Describe how you could make approximately 5 g of a 3% (w/w) ferrous ammonium sulfate solution
You want 3% w/w, which means 3g of the salt in 100 g soln (3g salt + 97g H2O).
You want only 5g and not 100 g soln. Therefore, 3g salt x (5g soln/100 g soln) = 0.15 g of the salt and you would make the total mass to 5g (which is 5.0-0.15 = 4.85 g H2O)