You will need to use the compound growth formula - this is...
final number = starting number * (multiplier ^ number of years)
We can turn the 6% into a multiplier by dividing it by 100 and adding 1 on to it:
6/100 = 0.06
0.06+1 = 1.06
Then we can say that the starting number is 254 and the number of years is X, we can then place this all into the original formula:
Final population = 254 * (1.06 ^ X)
Hope this helps! :)
