Alex wants to have $2500 in his bank account after 2 years. if the account earns 5% interest compounded 2 times per year, how much should he put in? round to the nearest cent.

You'll need to use this formula:
Principal = Total / [(1+ (rate/n))^n*years]
Where "n" is the number of compounding periods per year.
Principal = 2,500 / [(1+ (.05/2))^2*2]
Principal = 2,500 / (1.025)^4
Principal = 2,500 / 1.1038128906
Principal = 2,264.88

Source:
http://www.1728.org/compint3.htm

The variables should add up to equal 1