Let the weights of the cashews and peanuts be c and p respectively. Then:
c + p = 10 lb.
The related cost equation is 5.80c + 2.20p = 3.64(c+p).
Because c + p = 10 lb, the previous equation is equivalent to:
5.80c + 2.20p = 3.64(10) = 36.40
Solving c + p for c, we get c = 10 - p. Then, the last equation becomes:
5.80(10-p) + 2.20p = 36.40, or 58 - 5.8p = 36.4. Solving for p:
-5.8p = 36.4 - 58, or -5.8p = -21.6. Finally, p = 3.72 lb.
Since c + p = 10 (lb), c = 10-3.72 (lb), or c = 6.28 lb.
He must make this 10-lb mixture as follows: 6.28 lb of cashews and 3.72 lb of peanuts.