唔識 Exponential Growth or Decay

Please show all clear steps, thank you.

Ex1. The population pf P of a city in t years is predicted by the formula: P = 40000e^(0.05t)
a) In how many years will the population be doubled?

Ex2. If the world population is now 4.5 billion and if it has been growing continuously at an average rate of 2.3% per year. [P = (Pe^rt) base o]
a) How long will it take for the population to grow to 5 billion?

Ex3. The percent of carbon-14 of a dead plant or animal can be determined by the following equation. A = [Ae^(-0.000124t) base o]
a) Estimate the age of a bone if 30% of the original amount of carbon-14 was still present.

I totally had no idea how to do these, thank you so much! ^__^