for e.g in 2020 there are 3 significant figured. all zeros to the right of the last non zero digit are significant if they come from the measurement. for e.g if the distance between two points is measured 20w0cm (measured to the nearest centimeter) then in this number there are four significant figures
The rule is that any number 1,2,3,4,5,6,7,8,9 are sig figs. But 0, isnt a sigfig unless it is sandwidged between sigfigs, or to the right of a decimal point.
