Chapter 1 - Sections 1.1-1.8 - Exercises - Problems by Topic - Page 40: 76b

0.001$0$4$0$ m; cross first 3 zeroes and underline 2 last zeroes Significant zeroes above are bold.

First three zeroes are insignificant because they're leading zeroes (to the left of the first non zero digit) and only serve to locate the decimal point. The last two are internal and trailing zeroes, respectively.