Question

the radioisotope radon-222 has a half-life of 3.8 days. how much of a 65-g sample of radon-222 would be left after approximately 15 days?

Answer 1

Use the 1st order decay equation 
Cfinal=Cinitiale−kt 
by filling in and doing the math you will get:

 Cfinal = 1.11 grams Rn-222 after 23 days. 
 
hope this helps