How many significant figures does 15001000?

Answer 1

There are five significant figures. The two zeros between the 5 million and 1 thousand are significant: 15001000, because they are between other significant figures. They tell that those places have a certain value (other than 1,2,3 etc). The three zeros in the ones, tens and hundreds places are not significant, because they just indicate scale. Note that if it had said 15001000.0 then it could be said that those zeros are significant because they represent a specific value.

Answer 2

5 of them.