Provide your best answer for the following math problem. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The problem is: if the average ( arithmetic mean ) of a and b is 80 , and the average of b and c is 180 , what is the value of a − c ?
− 200
100.