Question 356726
Let x = # grams of water that has to be added.  Then 
{{{50*0.36 + x*0 = (50 + x)*0.20}}},
{{{18 = 10 + 0.20x}}},
{{{8 = 0.20x}}},
{{{40 = x}}}.  Therefore 40 grams of water has to be added.