If I understand this question correctly you are rounding whole numbers to return hundreds. The convention is usually to round up at the midpoint so 349 would round down to 300 and 350 would round up to 400. Likewise 449 would round down to 400 and 450 would round up to 500. Thus the whole numbers that round to 400 are all numbers between 350 and 449 inclusive. That is 449 - 349 = 100 numbers.Cheers,
After this answer was posted Mary Ann correctly noted that the number 400 doesn't need to be rounded so the correct answer should be 99 not 100.