Question 713346
price or percentage, either is just a kind of strength.  This is another example of a more general question.  Here, "concentration" means money, dollars.


ASSIGN VARIABLES TO ALL QUANTITIES
L, the lower concentration of available material, $0.80/pound
H, the higher concentration of available material, $1.80/pound
T, the target concentration of resulting mixture, $1.40/pound
x, the amount of lower concentrated material to use, unknown
y, the amount of higher concentrated materaial to use, 300 pounds



{{{highlight((Lx+yH)/(x+y)=T)}}}, only x is the unknown.  Solve for x, then plug in the given values.

{{{Lx+yH=T(x+y)}}}
{{{Lx+yH=Tx+Ty}}}
{{{Lx-Tx=Ty-Hy}}}
{{{x(L-T)=(T-H)y}}}
{{{highlight(x=(T-H)y/(L-T))}}}, substitute the values given into this formula.