# How can I perform the following operation 100-23, taking into account the number of significant figures?

##
Is the answer 100? 80? 77? Please explain!

Thank you!

Is the answer 100? 80? 77? Please explain!

Thank you!

##### 1 Answer

The answer is either 77 or 100, depending on whether the numbers were obtained by counting or by measurement.

#### Explanation:

**If obtained by counting**

All digits obtained by counting are significant.

Thus, **two** significant figures.

**If obtained by measurement**

Trailing zeroes are not significant in numbers without decimal points.

Thus, a measurement of

A measurement of

∴

The rules for **adding and subtracting** are that you must round off so the answer has no more digits after the decimal place than the number with the fewest digits after the decimal place.

Thus,

To the closest unit,

∴