Some persons consider as significant digits only 3 and 9; I think that is more correct to consider as significant digits 3,9,0,0,0.
Other propose to add bars or points (as 39000.) for distinction but this system is not practically used.
5
4
It has 3 significant digits.
3 significant digits.
It has two significant digits.
Five. All nonzero digits are significant and zeros in between significant digits are significant.
Five. All nonzero digits are significant and zeros in between significant digits are always significant.
Five. All nonzero digits are significant and zeros in between significant digits are always significant.
Five. All nonzero digits are significant and zeros in between significant digits are always significant.
Three significant digits.
50.4 has 3 significant digits.
There are 3 significant digits in 20.0