Greetings!
"Explain how to estimate 368+231 in two different ways."...
1) Add the numbers in the hundreds place and tens value together:
3+2=5
6+3=9
Roughly 590
2) Round the numbers to the nearest tenth place value:
368--->370
231--->230
Now add the numbers together:
37+23
=60
=600
Roughly 600
The actual answer is 599, so both techniques were pretty accurate.
Hope this helps.
-Benjamin