Question 188891
Let x=amount of the 40% solution
and let (x+1)=amount of 30% solution

Since we are only adding water, the amount of pure acid in the 40% solution (0.40x) has to equal the amount of pure acid in the 30% solution (0.30(x+1)), so:

0.40x=0.30(x+1) get rid of parens
0.40x=0.30x+0.30 subtract 0.30x from each side
0.40x-0.30x=0.30 collect like terms
0.10x=0.30 divide each side by 0.10
x=3 liters---------------amount of 40% solution
x=1=3+1=4 liters---------------------amount of 30% solution that she has

CK
0.40*3=0.30(3+1)
1.2=1.2

Hope this helps---ptaylor