Explain how to estimate 368+231 two differ ways

3 + 2 = 5

6 + 3 = 9

8 + 1 = 9

So the answer is 599

Second way:

round down the digits into hundreds, tens, and ones then get the sum of those.

300 + 200 = 500

60 + 30 = 90

8 + 1 = 9

500 + 90 + 9 is 599