Left to Right

School taught you to add right to left, carrying digits. But left-to-right addition is faster in your head. For 347 + 286: start with 300 + 200 = 500, then 40 + 80 = 120, then 7 + 6 = 13. Total: 633.

This works because the biggest digits come first. Even if you stop partway, you already know the answer is around 600. Right-to-left forces you to hold everything in memory until the final carry resolves — exhausting for mental math. Lightning calculators like Arthur Benjamin rely on this approach.