Notation
x= represent the volume for the 25% boric acid
y= represent the volume for the 50% boric acid
Solution
We can set up the following system of equationa. The first one for the boric acid:
[tex]0.25x+0.50y=30\cdot0.40=12[/tex]The second equation for the total volume is given by:
[tex]x+y=30[/tex]Let's solve for this last question x and we got: x= 30-y and if we replace in the first equation we got:
[tex]0.25(30-y)+0.5y=12[/tex]And solving for y we got:
[tex]7.5-0.25y+0.5y=12[/tex][tex]y=\frac{12-7.5}{0.25}=18[/tex]And finally solving for x we got:
[tex]x=30-18=12[/tex]So we need 12 L of boric acid at 25% and 18 L of boric acid at 50%
