Question 63246
If we have 50 grams of a 20% solution then 20% of the 50 grams is salt. 
20/100 x 50 = 10g of salt. This means we must have 40 grams of water.
The amount of salt in the mixture is x. So the amount of solution will therefore be x + 40
The fraction of salt in the mixture will be x/(x+40) which must equal 25%.
{{{x/(x+40)=25%}}} 25% is one quarter
{{{x/(x+40)=1/4}}} Multiply both sides by (x + 40)
{{{x/(x+40)*(x+40)=1/4*(x+40)}}}
{{{x=(x+40)/4}}} Multiply both sides by 4
{{{x*4=(x+40)/4*4}}}
{{{4x=x+40}}}
{{{3x=40}}}
{{{x=40/3}}}
{{{x=13.3}}}
The original mixture contained 10 g so an extra 3.3g of salt must be added.