Aperson wishes to mix coffee worth $9 per lb with coffee worth $3 per lb
to get 120 lb
of a mixture worth $5 per lb
how many pounds of the $9 and the $3 coffees will be needed?

To answer the question, let x be the number of pounds of coffee worth $9. Having said that, the number of pounds of coffee worth $3 is 120 - x. The equation relating the items before and after mixing is,
                      (x)($9) + (120 - x)($3) = (120)($5)
The value of x is 40. Therefore, the amount of coffee worth $9 and $3 to be used are 40 and 80 pounds, respectively.

The answer that you have written for 4 and 5 is correct