There are 2 sig. digs. The way I usually explain this one is to ask the question "Why would we bother to write the 0 after the 4 if it wasn't significant? After all 0.40 = 0.4!"Originally Posted bypocahontas

The reason the 0 before the decimal point is not significant is a little trickier to explain. Basically, it's just a place-holder. An example of why we don't count this digit as significant appears in a similar problem: how many sig. digs. does 4.0 have? We obviously wouldn't say 3 here because to write 04.0 is ludicrous (though the first 0 is, in fact, known with certainty). The reasoning for 0.40 is similar, though obviously much less trivial.

-Dan