Using Mental math, you can use the strategy of adding the hundreds place (300+200) and the tens (90+50), then the ones place (8+7), and then add these three results to get your answer.
Using the standard algorithm to follow the rules of adding numbers to find your answer.
For many the mental math method is much easier to use, even if you still have to write it down, it can be done much quicker than trying to carry remainders over, especially when the numbers have even more digits.