Answer
$\$20$
Work Step by Step
Let $x$ and $y$ be the servings of milk and pure whey respectively then we can write the equations for the given quantities as follows:
$32x+24y=640$......eq(1)
$240x+100y=3200$......eq(2)
We divide eq(1) by $8$ and eq(2) by $20$ to obtain:
$4x+3y=80$....eq(3)
and $12x+5y=160$.....eq(4)
Multiplying eq(3) by $3$ and subtracting eq(4) from it, we obtain:
$4y=80$
$\implies y=20$ that is there are $20$ servings of pure whey.
Now we plug in $y=20$ in eq(3) to obtain:
$4x+3(20)=80$
$\implies x=5$ that is there are $5$ servings of milk.
Thus, the cost of milk$=5\times 1.60=\$8$
The cost of pure whey $=20\times 0.6=\$12$
Total cost $=8+12=\$20$
