so 1000. is our four-significant-figure answer. (from rules 5 and 6, we see that in order for the trailing zeros to "count" as significant, they must be followed by a decimal. Writing just "1000" would give us only one significant figure.)
Consequently, which zeros are significant?
If a zero is found between significant digits, it is significant. Zeros can be used as (insignificant) place holders to the left of significant digits if the number is a decimal. For example, a mass of 42 g has two significant digits.
How many significant figures 50 has?
Trailing zero: Counts as a significant figure if the number contains a decimal point. However, it does not count if the number does not contain a decimal point. Example: 50.00 (4 significant figures), 50 (1 significant figure), 50. (2 significant figures).