Practice: compute 426 + 378 mentally using left-to-right addition.

804 — add hundreds, tens, then units into one running total.

426 + 378:

• Hundreds: 400 + 300 = 700
• Tens: 20 + 70 = 90, running total 790
• Units: 6 + 8 = 14, running total 804

You still handle the "carry" from 14, but it is absorbed into the running total instead of held as a separate mark.