Add from the left

a + b

The trick, step by step

1. Hundreds first: 300 + 200 = 500.

   500

2. Then tens: 40 + 80 = 120.

   120

3. Then ones: 7 + 5 = 12.

   12

4. Together: 500 + 120 + 12 = 632.

   632

given working answer taken away

Why it works

347 + 285 = (300 + 40 + 7) + (200 + 80 + 5), and the parts can be regrouped by place. Working from the left gives the biggest part of the answer first, which is also how you estimate.