First round the numbers to the nearest hundred.
339 --> 300
123 --> 100
Add them together.
300 + 100 = 400
Now add the tens places.
339 --> value of second 3 = 30
123 --> value of 2 = 20
30 + 20 = 50
Now add the ones.
9 + 3 = 12
Add all of these numbers together for your final number.
400 + 50 = 450
450 + 12 = 462
339 + 123 = 462
