211000000 = 1000000 * 0,9 ^ t
Como resolver isso?
1. Divide both sides by 1000000 to get 211 = 0.9^t
2. Take logs of both sides: ln(211) = ln(0.9^t) where ln is log to the base e (though you could use log to the base 10 if you prefer).
3. Use a property of logs: ln(211) = t*ln(0.9)
4. Rearrange: t = ln(211)/ln(0.9)
I'll leave you to evaluate (you can use the calculator on the Home page here).