Question 713345
Solve in general first, especially if you have other problems like that one.


ASSIGN VARIABLES TO ALL QUANTITIES
L, the lower concentration of available material, $0.40/pound
H, the higher concentration of available material, $1.30/pound
T, the target concentration of resulting mixture, $0.70/pound
x, the amount of lower concentrated material to use, 150 pounds
y, the amount of higher concentrated material to use,  UNKNOWN


{{{highlight((Lx+yH)/(x+y)=T)}}}
That can be partly changed by multiplying both sides by x+y to get  {{{Lx+yH=T(x+y)}}}.

{{{Lx+Hy=Tx+Ty}}}
{{{Lx-Tx=-Hy+Ty}}
{{{(L-T)x=(-H+T)y}}},  and since the differences in parentheses would both be negative,  we can multiply left side and right side members by -1 to get,
{{{(T-L)x=(H-T)y}}}
{{{x=y(H-T)/(T-L)}}}


In case x were known but y were the unknown, our solution would be the formula for y,
{{{highlight(y=x(T-L)/(H-T))}}}, THIS is what we want in this bird seed mix problem.



NOW substitute the known values from given in the problem description.
{{{highlight(y=150(1.3-0.4)/(1.3-.7))}}}, pounds of $1.30/pound sunflower seeds to add.