Polonium-218 is formed from the decay of Radon-222 and has a half-life of 3 min?

Question

What percentage of the polonium-218 will remain after 9 minutes? (The answer has to be in significant figures)

Please help me with this......

Answer 1

Easy as pie. In each "half-life", there is a decrease of half of the element. So 9 min/3min = 3 half-lives.
Then fraction remaining is (1/2)^3 or 1/8 = 0.125 or 12.5%