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Significant figures are a way to express the precision of a number. Understanding the rules for significant figures helps accurately report the certainty in scientific measurements and calculations. Here are the basic rules:

• Non-zero digits are always significant. These are numbers 1 through 9.
• Zeros between significant digits are also significant. They confirm precision in measurement.
• Leading zeros are not significant. They serve as placeholders and do not reflect measurement precision.
• Trailing zeros in decimal numbers are significant because they show precision in measurement. In whole numbers without a decimal, trailing zeros are not considered significant unless indicated by a bar or underlining.

By following these rules, you can accurately count the number of significant figures in any number.

Non-zero digits are the simple part of identifying significant figures. They are always significant because they inherently show measurement precision. For example, in the number 153.8, the digits 1, 5, 3, and 8 are all significant. They represent exact measured values or precise counts.

Always keep in mind that every non-zero number you see in a given value counts as a significant figure. These digits aren't affected by zeros that might surround them either before or after.

Leading zeros can be a bit tricky. These are zeros that appear before any non-zero digit in a number. Such zeros are not significant because they are needed only to position the decimal point.

For instance, in the number 0.000150, the sequence starts with four leading zeros. These do not count towards the number of significant figures. They are merely there to indicate the decimal placement and do not affect the precision of the number.

In contrast, if the number was written as 150 without the decimal, those leading zeros would vanish because they are non-significant.

Trailing zeros are those zeros that appear after the last non-zero digit in a number. Their significance can depend on whether the number is a decimal or a whole number.

For decimal numbers, trailing zeros are significant because they indicate precision. For instance, in the number 0.000150, the zero after the 5 is a trailing zero. It's significant because it confirms the precision of the measurement at three decimal places.

However, for whole numbers without a decimal, such as 1500, trailing zeros are ambiguous. They might not be considered significant unless the measurement context explicitly denotes their precision through notation, such as a bar over zero or using scientific notation to specify certainty.