how many pounds of each kind should he use in the new mix?

We have to find how many pounds of each mix we have to use in the new mix.
Let A be the pounds for the first mix, which is sold at $1.20 per pound.
Let B be the pounds for the second mix, which is sold at $2.35 per pound.
We then can use the information given to write a system of equations and solve for A and B.
We know that the total weight is 24 pounds, so we can write:
[tex]A+B=24[/tex]The price per pound for the mix is $1.65, so the total mix has to be sold at 24*1.65 = $39.60.
This price for the mix will be equal to the sum of the weight of each mix times its price per pound.
Then, we can write:
[tex]\begin{gathered} 1.20\cdot A+2.35\cdot B=1.65\cdot24 \\ 1.2A+2.35B=39.6 \end{gathered}[/tex]We can use the first equation to express A in function B:
[tex]\begin{gathered} A+B=24 \\ A=24-B \end{gathered}[/tex]Then, we replace A in the second equation and solve for B as:
[tex]\begin{gathered} 1.2A+2.35B=39.6 \\ 1.2(24-B)+2.35B=39.6 \\ 28.8-1.2B+2.35B=39.6 \\ 2.35B-1.2B=39.6-28.8 \\ 1.15B=10.8 \\ B=\frac{10.8}{1.15} \\ B\approx9.39 \end{gathered}[/tex]Knowing B = 9.39 we can calculate A as:
[tex]\begin{gathered} A=24-B \\ A=24-9.39 \\ A=14.61 \end{gathered}[/tex]Answer:
14.61 pounds of the $1.20 mix.
9.39 pounds of the $2.35 mix.