Question

Radon-219 has a half-life of about 4 s.

After 20 s, how many milligrams of a 500 mg sample will remain?

Answer 1

After 4 seconds, half of the initial amount is no longer Radon-219. After another 4 seconds pass, half of the amount remaining after 4 seconds is no longer Radon-219, leaving 1-1/2-1/4=1/4 of the original amount. Continuing removing half of the remaining amount every 4 seconds until a total of 20 seconds have elapsed. You'll have divided the original amount in half 5 times.

Answer 2

Here's a graph of the decay: