Question 814903
You want 9 dollars per kg to be 20% more than the price for the mixture.  
Some p mixture price before profit;
{{{9=1.20*p}}}
{{{p=9/1.20}}}
{{{highlight(p=7.5)}}}  dollars per kilogram, price of the mixture BEFORE profit.


The rest of the solution is just a standard two part mixture problem in which you treat prices like concentrations.


u is kg of the cheap peanuts and v is the kg of the expensive peanuts, and you can choose 100 kg. peanut mixture.


{{{(5.71u+8.5v)/100=7.5}}} and {{{u+v=100}}}
Solve for u and v.
The choice of 100 for the kilograms of the mixture was just arbitrary; you could choose a different value.  What you were asked was for the ratio between u and v.