Final answer:
Trailing zeros in a number are significant only if the number contains a decimal point.
Step-by-step explanation:
The number 100. has only 1 significant figure because trailing zeros at the end are significant only if the number contains a decimal point. Therefore, the zero after the decimal point in 100. is significant.
The number 100 has 3 significant figures because trailing zeros are insignificant when the number does not contain a decimal point. Therefore, the zero at the end of 100 is not significant.