Question 827575
p, c, and b, the amounts of peanuts, cashews, and brazil nuts.
{{{c=p-5}}} for the "five fewer" part.
{{{p+c+b=50}}} for the 50 pound wanted part.
'
{{{(2p+10c+10b)/50=6.80}}}  accounts for mixture price.


This appears to be a system of three equations in three unknowns, but proceed this way:


Multiply the mixture price account equation by 50 and then divide by 2 to simplify it.  Substitute for c in both the quantity sum equation and the simplified price account equation,and simplify these two; so you now have a system of two equations in just the unknowns, p and b.
Solve this simpler system for p and b; and then find the value for c.