Significant figures are digits in a number that gives its precision.
1. All digits in a number except for 0 are significant.
2. In numbers with a decimal point below 1, all zeroes after the decimal point but before the first significant digit are NOT significant.
3. Unlike in (2), the zeroes AFTER the last significant digit of a decimal are significant
4. If a 0 is put between any 2 significant figures, the 0 becomes significant.
5. It may be confusing if a number like 7500 is correct to the nearest hundred, or even only to the nearest unit as it may just happen to be a multiple of 100.
Here are some signs some people use to stop this confusion
5a. A bar may be placed either at the top or the bottom of the number indicating the last significant digit of that number.
5b. If that number is really correct to the nearest unit, then a decimal point may be placed at the end of it. like 100.
5c. Since these symbols may be unclear, it may be better to just write how many significant figures are there in the end of the number, abbreviated in s.f., as in 20000 (2s.f.)