If there were only 400, where do the extra 100 come from in option 2?
That said, if this genuinely confuses you there may well be others who are having similar problems and this should be noted in the example.
If there were only 400, where do the extra 100 come from in option 2?
That said, if this genuinely confuses you there may well be others who are having similar problems and this should be noted in the example.