Question 295960
I think you might be missing some info but let's continue.
p+c=100
3*p+4*c=100x
You are missing the final cost of the mix.
We have two equations and three unknowns
We can get it down to one equation and 2 unknowns. But we need at least one equation for each unknown.
3p+4(100-p)=100x
We do know that p is less than 100
and that x is between 3 and 4
x=4-p/100 
if p=60 then the cost of the mix will be $3.40
if p=40 then the mix will cost $3.60
The more peanuts the lower the cost
The more cashews the higher the cost