Exponential Growth and Decay

I'm having some trouble with the following problems:

1) Radioactive decay problem: What percent of a present amount of radioactive radium (226Ra) will remain after 900 years? (Half-life: 1599 years)

2) Radioactive decay problem: Find the half-life of a radioactive material if after 1 year 99.57% of the initial amount remains.

3) In 1990, the population of Mexico was 89 million and growing at the rate of 1.8% per year. In 1990, the US population was 250 million and growing at the rate of .7% per year. If these rates continue, when will the 2 nations have the same population?

Please help if you can, thanks alot.