Add by rounding

a + 38

The trick, step by step

1. Round up the awkward number: 38 is 2 less than 40.

   3840

2. Add the round number: 47 + 40 = 87.

   87

3. Take the extra back off: 87 − 2 = 85.

   − 285

given working answer taken away

Why it works

b = B − k, so a + b = a + B − k. Adding a round number B is easy, and then you correct for the k you added too many.