Question

Mayra did not have a date on a Friday night so she decided to mix solutions that she obtained from her garage. She has 9 liters of a 4% -saline solution that must be mixed with a 10% solution to get a 6% solution. How many liters of the 10% solution are needed?

Answer 1

Let $x$ be the number of liters of the 10% solution she needs to use. She'd end up with a 6% solution with a volume of $9+x$ liters. The starting solution contains 0.04*9 = 0.36 liter of salt. Each liter of the 10% solution contributes 0.1 liter of salt, so that

$0.36+0.1x=0.06(9+x)$

Solve for $x$:

$0.36+0.1x=0.54+0.06x$

$0.04x=0.18$

$x=4.5$

so Mayra needs to add 4.5 liters of the 10% solution.