The cost of cashews per pound = 10$
The cost of almonds per pound = 6$
1 pounds of peanut, 1 pound of cashews and 1 pounds of almonds are purchased for 20$ and is proved as 4 + 10 + 6 = 20$
While
2 pounds of peanut, 1 pound of cashews and 3 pounds of almonds are purchased for 36$ and is proved as (2*4) + (1*10) + (3*6) = 8 + 10 + 18 = 36$