The hundred that's nearest to 45 is zero.
45 is only 45 away from that one.
The next one ... 100 ... is 55 away.
round 90 to the nearest ten
372 rounded to the nearest hundred is 400
300
45 + 39 = 84, so if you want to round to the nearest ten, that's 80, or nearest hundred is 100. Otherwise, it doesn't turn out to a decimal, so it doesn't need rounding to the nearest one.
100 100
100
100
100
2,222 rounded to the nearest 100 is 2,200.
10100
100
4198 to the nearest 100 = 4200