The number of significant figures in the measurement of 45.030 mm is

Significant figures are all the digits excluding leading zeros for small decimals and zeros at the end of an integer.

45.030 has 5 significant figures because the zero at the end is after the decimal point. This means that the number was measured with 3 decimal places of precision. If the last zero was not significant, the number would be written 45.03.

On the other hand, 45030 only has 4 significant figures. Hope this helps!