#### Answer

2

#### Work Step by Step

We are given $0.0040 g$.
To find the number of significant digits, we follow the rules on page 162 of the book.
We use rule #1: all non-zero digits are significant. Thus "4" is significant.
We also use rule #4: all zeros to the right of a significant digit and decimal point are significant. Thus the "0" after the "4" is significant.
Finally, rule #6 tells us that the zeros to the left of the "4" are not significant.
Thus, there are 2 significant digits.