Question

how many significant figures are there in the number 100?

Answer 1

there is one significant figure as a significant figure is any number other than 0

if it was 120 then it would be 2

Answer 2

There is only one significant figure in 100. Remember the three rules that make a number significant:
1. Non-zero digits are ALWAYS significant
2. Any zeros between two SIGNIFICANT digits are significant as well.
3. A final zero or trailing zeros in the DECIMAL portion ONLY are significant.