I calculated this and it's wrong.
Wouldn't it be "if you have 29 generators, the next one would cost 331.06B" because you're calculating the 30th one (the one that gives you 331.06B)?
Also the formula is wrong. It's actually 5B•(1.15)²⁹ (raised to the power of 29), and not raised to the power of 20
5B•(1.15)²⁹ = 331.06B
5B•(1.15)²⁰ = 81.83B