The bank, like it says multiples adjacent offices 2.5 times. But thing change when you add more banks next to offices. The banks just adds the difference from the original change. For example, you have a 2 billion selling power office. You put a bank next to it and you now sell 5 billion power. A 3 billion difference. When you add the second bank that office only sells 8 billion power, not multiplying but adding the difference from the first time. This goes on for every bank added on.
When you are looking for the most selling power you tend to try to have a bank surrounding each office. Most of the time having more banks than offices are better. It should look like something like the picture down below.