Question 704475
Question is typical, typical, typical!  Rational expression equals a value.


Let x be how much water, y is how much of the 30%.
{{{y*0.30/(w+y)=0.10}}}
{{{x+y=30}}}


Solve one of the equations for x and substitute into the other equation and solve for y.  Now use either equation to solve for x value. 


You'll see that I did not put any "x" into the numerator of the rational equation.  Do you understand why?  ( x*0=0, the water by itself has no fertilizer ). 


Since you know 30 liters is the amount of mixture desired, the rational equation can be written, {{{y*0.30/30=0.10}}}, and in this case is simpler.