Because the number increase by a percentage this is an exponential growth problem.
The equation for exponential growth is :
Total = start value x ( 1 + percent) ^ time
Using the information from the problem:
2011-2005 = 6 years.
Total = 23 x (1 + 0.72)^6
Total = 23x 1.72^6
Total = 595.52 = 596 total people