When we round off a number to the nearest 100, we check the last two digits of the number. If the last two digits form a number less than 50, we round down the number to the nearest lower multiple of 100. If the last two digits form 50 or more than 50, we round up the number to the nearest higher multiple of 100.
Since the number 3255 has to be rounded off to the nearest hundred, our options are 3200 or 3300. 55 > 50.
Hence the length of River Ganga when rounded off to the nearest hundred will be 3300 km.