It depends upon how you got to the 12, for example:
28.6 - 16.6 = 12.0
Because each of the numbers that was used to get to the 12 had 3 significant figures, you should write the 12 with three significant figures also.
However:
29 - 17 = 12
In this case, each of the numbers that was used to get to 12 had only 2 significant figures, so use only 2 significant figures in the 12.
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Only three significant digits are correct, because 5.0 contains no digit for hundredths, and such a digit would be required to give four significant digits in the answer. Because 7 + 5 equals exactly 12, a third significant digit is gained for the answer. Note in contrast that the sum of 7.68 and 0.50 would properly contain four significant digits.
Four - zeros between significant digits are significant.
All of the digits are significant:Any digit after the decimal point is significant.A zero is significant if it comes between other significant digits.
There aren't varying degrees of significance - either it's a significant digit or it's not. Nine and two are significant and the two zeros aren't.
To one significant digit, 37.19 is 4x101. Writing it as 40 makes it unclear as to whether you have 1 significant digit or 2.