How many significant digits does 520 have?
Right from the start, you should be able to look at this number and say that it has at least two significant figures given by the two non-zero digits it contains.
Non-zero digits are always significant, so look for them first when trying to figure out how many sig figs you have.
Now, let's focus on that zero.
A zero that follows all non-zero digits and is not followed by any non-zero digits is called a trailing zero. A trailing zero is significant when it follows a decimal point or when a decimal point directly follows it.
In your case, the zero in
Examples in which that zero would be significant
#52.0 ->#the trailing zero follows a decimal point
#520. ->#the trailing zero is followed directly by a decimal point