thanks for your help.it solved

round off will solve the problem

Yes I think so , writing problem.

You need to do that every year, not only at the end. If you only do that at the end, you'll get the wrong result. akar-0 already told you that:

You must round down for each year.

And that's what this shows:

At the end of the first year there will be: 
1000 + 1000 * 0.02 + 50 => 1070 inhabitants

At the end of the 2nd year there will be: 
1070 + 1070 * 0.02 + 50 => 1141 inhabitants (** number of inhabitants is an integer **)

At the end of the 3rd year there will be:
1141 + 1141 * 0.02 + 50 => 1213

It will need 3 entire years.

Yes there's a problem with the kata's writing. I was wondering why I was getting that error. I have try to debug it but to no avail.

There is no fractional population, read the kata's description again.

You must round down for each year.