875
15.24
All its digits are decimal digits so the question makes no sense.
3 is the ones digit in 4903. Since 3 is less than five, the ones digit becomes zero when 4903 is rounded to the nearest 10. 4903 to the nearest 10 is 4900.0 is the tens digit in 4903. Since 0 is less than five, the ones and tens digit are each 0 when 4903 is rounded to the nearest 100. 4903 to the nearest 100 is 4900. 9 is the hundreds digit in 4903. Since 9 is five or more, the thousands digit next to it is increased by one and the hundreds, tens, and ones digits are each zero. 4903 to the nearest 1000 is 5000.
199
2722
500
The answer depends on which digit is 5 and what the other digits are!
10 (565,566,567,568,569,570,571,572,573,574)
875 rounded to nearest ten is 880 8+8+0 is 16
555
2.56
835
-12
3400 is already at the nearest 100. Rounding to the nearest hundred means that the last two digits (tens and units digits) are replaced by 00 if they are between 1 and 49 (for example 5419 would be rounded to 5400), and if the last two digits are between 50 and 99, then the hundreds digit is increased by 1 followed by 00 (so 2456 would be rounded to 2500).
33100
875
226