Given:
The cost of mixture per pound, M= $5.91.
The quantity of nuts taken first , n=13 pounds.
The cost of nuts per pound, which was taken first, N=$3.10.
The cost of nuts per pound taken second, P=$7.50.
Let p be the quantity of nuts taken second.
Hence, we can write,
[tex]M(n+p)=Nn+Pp[/tex]Now, put the values in the above equation.
[tex]\begin{gathered} 5.91\times(13+p)=3.1\times13+7.5\times p \\ 5.91\times13+5.91p=3.1\times13+7.5\times p \\ 5.91p-7.5\times p=3.1\times13-5.91\times13 \\ -1.59p=13(3.1-5.91) \\ -1.59p=13\times(-2.81) \\ -1.59p=-36.53 \\ p=\frac{-36.53}{-1.59} \\ p\cong23\text{ pounds} \end{gathered}[/tex]Therefore, 23 pounds of nuts that cost $7.50 per pound is added to make the mixture.