15 is your answer as its 5*3
rules to find :- 1 All non zeroes number are significant
2 if zero proceeding the decimal point also work as non zero figure
3 zero are following the decimal point and proceeding the first non zero digit
4 Zero in between 2 non zero digits are also significant
5 if 1 zero follow the decimal point
790 :- 2 significant figures
0.0391 :- 3 significant figures ( follow 3 rule)
81.05 :- 4 significant figures ( rule 4)
0.000001 :- 1 significant figure
8546 :- 4 significant figures
79.0 :- 3 significant ( rule 5)
