2.13 + 1 = 3
When adding values, you must round your answer to the same "place" as the measurement with its last significant figure furthest to the left.
2.13
+1
3.13
The “1” has its last significant figure furthest to the left, so you round off the number to the “units" place. That's the last column that the measurements share.
3.13 → 3 (rounded to the "units" place)
∴ 2.13 + 1 = 3
