Question 762279
{Amount of pure acid} / {Amount volume of mixture} = {Concentration of the acid}


{{{(40*x+15*20)/(x+20)=25}}}  and your equation is also good.  


You asked if your resulting answer is good.  TEST it in your equation!  If you find equality, then your number result is correct, otherwise not.


Working a solution for  x from YOUR equation:
{{{0.4x+3=0.25(x+20)}}}
{{{0.4x+3=0.25x+5.0}}}
{{{(0.4-0.25)x=5}}}
{{{0.15x=5}}}
{{{x=5/0.15}}}
{{{highlight(x=33&1/3)}}}