Suppose a laboratory has a 61 g sample of polonium-210. The half-life of polonium-210 is about 138 days.
a. How many half-lives of polonium-210 occur in 828 days?
b. How much polonium is in the sample 828 days later?

6; 0.48 g
6; 10.17 g
6; 0.95 g
6; 4,209 g

Question

Suppose a laboratory has a 61 g sample of polonium-210. The half-life of polonium-210 is about 138 days.
a. How many half-lives of polonium-210 occur in 828 days?
b. How much polonium is in the sample 828 days later? 
  
 6; 0.48 g 
 6; 10.17 g 
 6; 0.95 g 
 6; 4,209 g

jim_thompson5910 · Answer

1 halflife = 138 days
2 halflives = 2*138 = 276 days
etc etc

x halflives = 138x days

so you need to solve

138x = 828