We have to calculate the average cost per unit of manufacturing h more items.
We first calculate the total additional cost:

To calculate the average cost, we divide the total additional cost by the number of additional units h:

If the level of production is x=100, we can write:

Then, for h=10 we have:

When h=1, we have:

When h tends to 0, we can calculate:

Answer:
The average unit cost of h=10 more units is 70.10989.
When h=1, the average unit cost is 70.10999.
The average unit cost of an additional unit, at the level of production x=100 (also known as "marginal unit cost" C'(x)) is 70.11.