Răspuns :

stim ca ms = md+m.apa

si c% = mdx100/ms

=> md = msxc%/100

= (md+m.apa)20/100

=> 100md = 20md + 20m.apa

=> 80md = 20x400

=> md = 100 g

facem proba

ms = 100+400 = 500 g sol.

md = 100 g

=> c% = 100x100/500 = 20% deci verifica