If you set you the equations such as 3c+5w=23 and 6c+2w=20 and then solve the first equation for c, you get c= (23-5w)/3 Taking that equation and substituting it for 'c' in the second equation (so the second equation becomes: 6((23-5w)/3)+2w=20 ) you get w=3.25 If you plug that back into your first equation (so it now becomes: 3c+5(3.25)=23 ) you get c=2.25 So the answer would be walnuts cost $3.25 per lb and choc. chips cost $2.25 per lb.
