Let x represent the amount of pure salt(100%) added...
Mixing to form (24kg+x)of 40% solution
.20*24kg + x = .40(24kg+x)
.60x = .20*24kg
x = .20*24kg/.60
x = 8kg, the amount of pure salt that needs to be added
CHECKING our Answer***
4.8kg + 8kg = 12.8kg = .4*32kg = 12.8kg