Average fixed cost decreases as the number of output increases. Hence, if a firm spent $20 to produce 5 bags of rice, when it increases the output level to 10 bags of rice the cost will not change because it is a fixed cost, but rather, the same amount of fixed costs will be spread over a larger number of units of output.
Hence, the $20 cost that was used to produce 5 bags of rice, will accommodate the new level of output.
If 5 bags were produced at $20
Then 10 bags will also be produced at $20 cost
Hence we have \(\frac{20}{10}\) = 2
$2 is the average fixed cost per unit of producing 10 bags