Answer: 20 pounds of peanuts and 80 pounds of cashews.
Although the question is incomplete, I have seen it before.
The equations are:
3a + 6b = 12
-3a + 6b = -12
Adding the equations, we get:
12b = 0
And b = 0
The pattern is multiplying by 3. So it will be: 243, 729
