It takes 5.2 minutes for a 4.0 g sample of francium-210 to decay until only 1.0 g is left. What is the half-life of francium-210? 7.8 minutes 1.3 minutes 2.6 minutes 5.2 minutes

This problem is not about chemistry it is about math. the half-life of a certain thing is the time that it needs to decay 50% of what it had before. so if there is 4g, after 1 half there will be only 2g. so to become 1g it is passed 2 half-lives. 4,0g, 2,0g, 1,0g so it is the time divided by 2 (numbers of half-lives). So the half-life of Francium 210 is 2,6 min.