Question 125173
Start by letting x = the number of ml of 5% hydrochloride acid solution required to be added to the 120 ml of 20% hydrochloride acid solution to get a total of (120+x) ml of 10% hydrochloride acid solution. 
We can write the equation to solve for x:
{{{0.05x + 0.2(120) = (120+x)(0.1)}}} Simplify and solve for x.
{{{0.05x+24 = 12+0.1x}}} Subtract 0.05x from both sides.
{{{24 = 12+0.05x}}} Subtract 12 from both sides.
{{{12 = 0.05x}}} Finally, divide both sides bt 0.05
{{{240 = x}}}
The student will need to add 240ml of 5% hydrochloride acid solution to 120ml of 20% hydrochloride acid solution to obtain 360ml of 10% hydrochloride acid solution.