Question

the half-life of bismuth-210 is 5 days. if 25 grams of bismuth-210 are present initially, how much will remain after 10 days?

Answer 1

10 days is twice the half-life. After 5 days, can expect $\dfrac{25}2$ grams to remain; after another 5 days, you'd be left with $\dfrac{25}4$ grams.