They add up to 807
The tens place is two digits before the decimal point. To round it use the ones digit (left of the decimal point) to decide whether to round up (ones digit is 5 or more) or down (ones digit is 4 or less). In 907.457 the ones digit is 7 ehich is 5 or more, so it rounds up to 910.
They add up to 72
The answer would be 37.
The answer depends on what the tens digit is greater than, and what the ones digit does then.
Assuming that "adds up to" means "its digits add up to" 888
Add together the like terms. 5 thousands and 4 thousands makes 9 thousands. 3 hundred is a given. 2 tens is a given. 4 ones and 2 ones is 6 ones. You end up with: 9 thousand, 3 hundred, 2 tens, 6 ones. Just write down the numbers in order. "9326"
660. remember when rounding to the nearest ten your lowest non zero number will be in the tens place and you will look at the ones place to decide whether or not to round up. If the ones place is 5 or above, round up in the tens place. If the ones place is 0-4 keep the remaining tens place number.
Roman numerals are not appropriate for doing even simple operations such as an addition. Pressumably, the ancient Romans had to use some other system, or perhaps an abacus, to do the actual addition. You can try using basically the same system you use for Arabic (i.e., our standard) numbers: add the ones and ten places separately; for the ones place in the result, split it up into tens and ones, and add the tens to the other part of the result.