78
312
6999999
213
555
555
199
-12
The mystery number is 155. When rounded to the nearest hundred, it rounds to 200. The sum of its digits is 1+5+5=11 which is greater than 10, so we must adjust the digit in the hundreds place. When rounded to the nearest ten, 155 rounds to 160, which has digits summing up to 1+6+0=7.
199 If it's 200 rounded to nearest ten, then the number must be between 195 and 205. Only 199 has the correct digit sum.
To round to the nearest hundred and ten both being 200 it leaves 9 possibilities. 195,196,197,198,199,201,202,203, and 204. The only one with digits adding up to 19 is 199.
No reason. The nearest ten will be closer to the actual sum.
875 rounded to nearest ten is 880 8+8+0 is 16