A 40-g solution of concentration 28% salt contains
0.28 (40 g) = 11.2 g
of salt.
To get a solution with concentration 40%, suppose you add x grams of salt. Then this new solution has a new total mass of (40 + x) g and contains (11.2 + x) g of salt. At 40% concentration, we have
(11.2 + x)/(40 + x) = 0.40
Solve for x :
11.2 + x = 0.40 (40 + x)
11.2 + x = 16 + 0.40x
0.60x = 4.8
x = 8
