The number $2.1390$ has five significant figures.

Here’s why:

• All non-zero digits are always significant.
• Any zeros between non-zero digits are significant.
• Any trailing zeros after a decimal point are significant, as they indicate precision in the measurement.

In $2.1390$:

• The digits $2$, $1$, $3$, and $9$ are all significant.
• The trailing zero is also significant because it is after the decimal point.

Would you like further details or have any questions?

Here are some related questions:

1. How many significant figures are in $0.004520$?
2. What are the rules for determining significant figures?
3. How do significant figures affect the accuracy of a calculation?
4. Why is the number of significant figures important in scientific measurements?
5. How do you round a number to a certain number of significant figures?

Tip: When performing calculations, the result should be reported with the same number of significant figures as the measurement with the fewest significant figures.