Hey!
ms1 = 80 g
c1 = 10%
ms2 = 200 g
c2 = 30%
mH2O = 50 g
cfinal = ?
--------------------------
md1 = c1*ms1/100
md1 = 10 * 80/100
md1 = 8 g
md2 = c2*ms2/100
md2 = 30*200/100
md2 = 60 g
mdfinal = md1+md2
mdfinal = 60 g+ 8g
mdfinal = 68 g
msfinal = ms1+ms2+mH2O
msfinal = 80g+ 200g+50g
msfinal = 330 g
cfinal = mdfinal*100/msfinal
cfinal = 68*100/330
cfinal = 20,6 %.