Let a be the amount of peanuts; and
Let b the amount of walnuts
Equation 1 would be: A + B = 100
249a + 389b = 319 (100)
Equation 2 would be: 249a + 389b = 31900
From equation 1:
a = 100 – b
249 (100 – b) + 389b = 31900
24900 – 249b + 389b = 31900
140b = 7000
b = 50 lbs
then from equation 1:
A + 50 = 100
A = 50 pounds