To find out how many significant numbers there are in a number we can rely on a few rules:
1. Non-zero digits are always significant
2. Any zeros between non-zero digits are significant
3. Final zeros or trailing zeros are significant if they come after the non-zero numbers in a decimal number situation
Example: .500 and .632000
In our question we have:
12.3 + 0.005675
Our final value will have 3 significant numbers because of 12.3, and since only 3 significant figures will be at the end, the result will be:
12.3 + 0.005675 = 12.3