Question 164105
Let x=amount of water needed

Now we know that the amount of pure acid in the 60g solution (0.70*60) plus the amount of pure acid in the water that's added(0) has to equal the amoount of pure acid in the final solution (0.40(60+x)).  So our equation to solve is:

0.70*60=0.40(60+x) get rid of parens (distributive law) and simplify
42=24+0.40x  subtract 24 from each side
42-24=24-24+0.40x  collect like terms

18=0.40x  divide each side by 0.40
x=45g-------------------------------------amount of water needed

CK
0.70*60=0.40*105
42=42

Hope this helps---ptaylor