The closest hundred to 29 is zero. You can think of it in terms of money. If you have 29 cents and you want to know approximately how many dollars you have, the answer is none.
100
30
593 656 rounded off to the nearest hundred is 593 700.
409 to the nearest hundred is 400
To round off 78 to the nearest hundred, you look at the tens digit, which is 7. Since 7 is greater than 5, you round up. Thus, 78 rounded to the nearest hundred is 100.
409,381,900
10100
100
600
900
2,600
3600