Originally Posted by **ThePerfectHacker**

Here is the formula,

$\displaystyle \mbox{concetration}=\frac{\mbox{amout of substance}}{\mbox{total amount present}}$

You have 20 milliletrs of 10% vineger solution. Meaning 10% of 20 is vineger thus, 2 milliletrs of vineger. You add $\displaystyle m$ millileters to get a concentration of 25% meaning,

$\displaystyle \frac{2+m}{20+m}=.25$

Note, and understand why the numerator is the way it is and the denominator is the way is it.

Now cross mutiply considering .25 as a fraction 1/4 thus,

$\displaystyle 4(2+m)=20+m$

Thus,

$\displaystyle 8+4m=20+m$

Subtract 8 and m from both sides,

$\displaystyle 3m=12$

Thus,

$\displaystyle m=4\mbox{ millilters}$ need to be added.