Let x = number of lb. of pecans
y = number of lb of cashews
x + y = 48 This represents the total amount of the mixture in lb.
3x + 6y = 48(4) This represents the total cost of the mixture
3(x + y = 48)
3x + 6y = 192
We multiply the top equation by 3, and then subtract the two linear equations from each other to get:
3x + 3y = 144
3x + 6y = 192
-3y = -48
y = 16 lb of cashews
We know that x + y = 48
So x + 16 = 48
x = 32 lb of pecans