Question 41835
Let us suppose that 20 pounds of peanuts is mixed with 'x' pounds of cashew.


Cost of 20 lbs. of peanuts = $(3x20) = $60.
Cost of cashew nuts = $5 per pound.
So cost of 'x' pounds of cashew = $5x.
So the cost of the mixture of 20 lbs. of peanut and 'x' lbs. of cashew = $(60+5x).
In other words cost of (20 + x) lbs. of the mixture = $(60+5x).
So, cost of each pound of the mixture = ${{{(60+5x)/(20+x)}}}.
Given, the price of this mixture is $3.5 per pound.


So, {{{(60+5x)/(20+x) = 3.5}}}
or {{{(60+5x) = 3.5(20+x)}}}
or {{{60+5x = 70 + 3.5x}}}
or {{{5x-3.5x=70-60}}}
or {{{1.5x = 10}}}
or {{{x=20/3}}} 
or x = 6.67 (approx)


So 6.67 lbs. of cashew is to be added.