Make a ten or hundred to add mentally 339+123

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