608,909 is the exact number as these people visited the monument in September month after the first week. To know the exact number of visitors you need to do to subtract 160,747 from 769,656 because you're trying to remove the number of visitors in the first week to find the visitors in the following 3 weeks.
Here,
769,656 - 160,747
= 608,909
Hence, through the easy subtraction method we get the exact number of people who visited the monument.