M.MgSO4*7H2O = 120+126 = 246 g/mol
c% = mdx100/ms
ms = m.cristaloh. + m.apa
md = m.MgSO4.anhidru
30 = 100md/(123+m.apa)
246g cristaloh. ......................... 120 g MgSO4
123 g cristaloh. ......................... md = 60 g
=> 30(123+m.apa) = 100x60
=> m.apa = 77 g apa
V.apa = 77 mL