The first thing that you need to know is how count the number of significant digits and how to find the estimated digit in a number.
So the explanations make sense, digits fit into one of two categories.
nonzero digits: 1, 2, 3, 4, 5, 6, 7, 8, 9
zeros: 0
Writing decimals.
When writing number less than zero, place a zero before
the decimal place. This zero indicates that the dot on the paper is a decimal and not a random mark on the document. (It's not related to significant figures, it is point of style.) Three hundredths looks like this, "0.03" and not, ".03." Fiftysix thousandths looks like this, "0.0056" and NOT ".0056."
Numbers that are called "significant" are know with complete accuracy. It also understood that the last number in a string of significant figures is the estimated number.
In dealing with significant figures, a large part of identifying them is dealing with zeros. Here 6 rules to help deal with the role of zeros when counting and doing math operations with significant figures.
1. 
Nonzero numbers are all significant. The last number is the estimated measurement. The other numbers are known with exact precision. 

Examples 
432 
3 sig. figs. 
432:The 2 is estimated. 
945.62 
5 sig. figs. 
945.62: The 2 is estimated. 
58 341 
5 sig. figs. 
58 341: The 1 is estimated. The three zeros are significant. 
245 379 
6 sig. figs. 
245 379: The 9 is estimated. 
92.93817 
7 sig. figs. 
92.93817: The 7 is estimated. 
685 721 394 
9 sig. figs. 
685 721 394: The 4 is estimated. 

2. 
Nonzero digits and zeros between nonzero digits are always significant.
In other words, zeros that are surrounded by nonzero numbers, are counted as significant.
In the number 2406, the 4 and the 6 are surrounding the 0. This make the zero significant. In the number 9380, the zero is not surrounded and is therefore no significant. Surrounding a zero by a nonzero number and a decimal on the RIGHT is the same as surrounding the zero by two nonzero numbers. "9380." means the zero is significant because of the number 8 on the left of the zero and the decimal is to the RIGHT. A decimal point to the left does not create significance. This is covered in a later rule.


Examples 
501 
3 sig. figs. 
501:The 1 is estimated. The zero is between nonzero digits and it significant. 
940.02 
5 sig. figs. 
940.02: The 2 is estimated. The two zeros significant too. 
50 001 
5 sig. figs. 
50 001: The 1 is estimated. The three zeros are significant. 
600 309 
6 sig. figs. 
600 309: The 9 is estimated. The zeros are all between other nonzero digits and are therefore significant. 
62.03007 
7 sig. figs. 
62.03007: The 7 is estimated. The zeros are significant because they are between other nonzero digits. 
3021 
4 sig. figs. 
3021: The 1 is estimated. The zero is significant. 
60. 
2 sig. figs. 
60. : The 0 is estimated. The zero is surrounded by the 6 and the decimal ON THE RIGHT. A decimal on the left does not count. 



How many significant figures are in each number shown below? 
23.01 

6003 

409 

35.002 

804 005 

93821 

500.01 

300 403 

900.2 

508 

567 

30.0782 







Which digit in the number below is the estimated measurement? 
56.08 

35.02 

63.04 

30.006 

347.86 

900 004 

8907 

596 

60 409 

45 



