Question 148915
C(s)=5s
P(t)=2t

where s is the amount of pounds of cashews, and t is the amount of pounds of peanuts.

We want 
(C(s)+P(t))/(s+t)=3

and

s+t=21

thus,

C(s)+P(t)=63
5s+2t=63.
s+t=21--->s=21-t

5(21-t)+2t=63
-3t=-42
t=14

If t=14, 
5s+28=63
5s=35
s=7

So we want a mixture of 7 lb of cashews and 14 lb of peanuts in order to make 21 lb of mixture that will sell for $3./lb.


Check:
7*5=35
2*14=28
35+28=63

$63/21 lb=$3/lb