Answer
7.2 L
Work Step by Step
Set up a table:
strength || liters of soution of pure acid ||
100% || x || x
30% || 18 || 5.4
50% || x+18 || 0.5 (x+18)
The equation to solve the problem is:
$x+5.4=0.5(x+18) $
multiply the equation by 2 to make the x value on the right side a whole number instead of a decimal
$2x+10.8=x+18$
then transfer values to each sides
$2x-x=18-10.8$
subtract each sides
$x=7.2$
