ACS Organic Chemistry Practice Test

Significant figures express how precisely a measurement is known. In 50.0, the three digits shown are all meaningful: the 5 is nonzero, the zero in the ones place is significant because the decimal point indicates precision reaching the tenths place, and the final zero in the tenths place is significant as well. So there are three significant figures. If it were written as 50 without a decimal, the trailing zero could be just a placeholder, giving only two significant figures.