Just as in base 10, when you get to the highest single digit and add 1, you carry into the next higher place value. If your number were 129 in base 10, the next number in sequence is 130. If your number were 1399, the next number in sequence would be 1400.
[tex]124_5+1=130_5\\\\1344_5+1=1400_5[/tex]
