If you want to have $12,000,000 at retirement. You start to deposit $40,000 per year at 12% into your retirement account. How many years will it take you to have that amount? And how do you calculate this information to get that answer?

31 Years.

The brute force way is like this - find N such that:

40,000 * [ (1+i) + (1+i)^2 + (1+i)^3 +... + (1+i)^N] >= 12,000,000

You can set up a pretty simple spreadsheet to see how large N has to be to satsify the condition.