When this 3-digit number is rounded to the nearest ten or hundred, the numbers round up. all of the digits in this number are the same. the sum of the digits is 15. what is the number

I don't need to know anything about all that rounding stuff.

You said that there are 3 digits, they are all the same, and their sum is 15.

So each of them must be  ( 15 / 3 )  =  5 .

The number is  555 .

the law of definite proportions states samples of a compound will always contain the same proportion of elements by mass. in other words, a given compound will always contain the same elements in the same proportions by mass.the law of multiple proportions states when two elements combine with each other to form more than one compound, then the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers.