Question 605692
For a word problem, the first step is to determine what it is we are trying to solve for.  In most cases, once we figure this out we can give it a variable and using the question make an equation using that variable.  

In our case, we are trying to solve for the number of litres of the 30% solution.  Let's call this x.

Using the question, we can write the following equation:
{{{x(0.30)+2(0.12)=(x+2)(0.15)}}}
(This is because we have x litres of 30% solution, adding that to 2 litres of 10% solution should give us x+2 litres of 15% solution-- if this is unclear please email me and I can help you further:) )

So now all that is left to do is solve for x:
{{{0.3x+0.24=0.15x+0.3}}}
{{{0.3x-0.15x=0.3-0.24}}}
{{{0.14x=0.06}}}
{{{x=0.06/0.14}}}
{{{x=4.29}}}
This means that there needs to be approximately 4.29 litres of the 30% acid solution mixed.