The population of a city is growing at an average rate of 3% per year. in 1990, the population was 45,000. determine the year during which the population will have doubled.

45000 * 1.03^t = 90000 

1.03^t = 2 
t*log(1.03) = log(2) 
t = log(2)/log(1.03) = 23.44977.. 

This means that the population would have doubled in 23.44 years i.e. roughly in the middle of 2013, so it will first have exceeded 90000