Question 79808
Let x=amount of water that must be added

Now we know that the amount of pure water added (x) plus the amount of pure water in the 50 liter acid solution (50)(0.70){if it's 30% acid, then it must be 70% water} must equal the amount of pure water in the final mixture (50+x)(0.80).  So our equation to solve is:

x+(50)(0.70)=(0.80)(50+x)  get rid of parens

x+3.5=4+0.80x   subtract 0.80x and also 3.5 from both sides

x-0.80x+3.5-3.5=4-3.5+0.80x-0.80x  collect like terms

0.20x=0.5  divide both sides by0.20

x=2.5 liters-------------amount of pure water that needs to be added

CK

2.5+3.5=4+2
6=6


Hope this helps---ptaylor