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. Expressed in kilograms, the mass of 0.042kg should still have two significant digits. The zeros to the left of the 4 are place holders and not significant. significant.
If the last significant figure(s) is (are) zero, life gets a whole lot more complicated. Suppose I used a metric ruler with millimeter markings to measure the width of a skateboard. The skateboard, a precision model, measures exactly 20 centimeters, and I report the width as 20cm$\pm$ 0.02mm. The uncertainty in this number is in the fourth digit (the hundredth's place), so it has 4 significant digits, not just 1. If I write out the 4 sig-digs, the width is 20.00cm. Since the zeros to the right of the decimal place are not necessary as place-holders, their inclusion indicates they are significant.
If the last significant figure(s) is (are) zero, life gets a whole lot more complicated. Suppose I used a metric ruler with millimeter markings to measure the width of a skateboard. The skateboard, a precision model, measures exactly 20 centimeters, and I report the width as 20cm$\pm$ 0.02mm. The uncertainty in this number is in the fourth digit (the hundredth's place), so it has 4 significant digits, not just 1. If I write out the 4 sig-digs, the width is 20.00cm. Since the zeros to the right of the decimal place are not necessary as place-holders, their inclusion indicates they are significant.
