The present population of the country is 15.2 million = 15.2*10^6. The population has grown at 2.7% per year for 5 years.
Let the population five years ago be P.
P*(1.027)^5 = 15.2*10^6
=> P = 15.2*10^6/(1.027)^5
=> P = 13.3*10^6
The population 5 years ago was 13.3 million
@justaguide thanks but I'm supposed to be using series and sequences notation, like U1, U5.. And I'm supposed to be solving with the geometric sequence formulas such as Un = U1 * r^n
r is the ratio by which you multiply a term to get the one after it in a geometric sequence