Application of Equations

Let x g and y g be the mass of 10% and 5% salt solution added respectively
x+y=400 ----- (1)
10%x+5%y=400(8%)
10x+5y=3200
2x+y=640 ----- (2)

(2)-(1): x=240
Put x=240 into (1)
240+y=400
y=160

∴ 240g of the 10% salt solution and 160g of the 5% salt solution should be added.