Then supposing we don't expand for a while, then at times D, E, F.... we still need to extract those 8 units of oil, but need to manufacturer 0 more batteries.

A bit abstract, but I couldn't move on without writing it out.

3/3

Now if we have at time A, 5 machines that run off 5 units of battery, we extract 5 units of battery.
At time B, expand by 3 machines, so extract 3 units of battery to accommodate.
At time C, we don't expand, so we don't need more battery.

Total battery extraction: 8
Total oil extraction: 21

2/

My understanding is:

At time A, we have 5 machines that run off 5 units of oil, so we extract 5 units.
At time B, we expand by adding 3 machines for 3 units of oil, so we now extract 8 units to keep everything running.
At time C, we don't expand, but still need to extract 8 units of oil.

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