Question 712924
p for pounds peanuts to add to original mixture, 3 pounds
v for pounds original mixture, 15 pounds

H for percent peanuts percent, 100%
L for percent peanuts in the mix before adding the 3 pounds of it, 25%
T UNKNOWN % peanuts in the mix after adding the 3 pounds of it
M = 3+15=18 pounds of final mixture


Formula for T is straight-forward.
{{{highlight(T=(H*p+L*v)/(M))}}}
We know everything to calculate and find T.  


Percent Peanuts = {{{(100*3+25*15)/18}}}  %
={{{37.5}}}% peanuts