How would you calculate the time it would take a fraction of a sample to decay given its time of half-life?

Question

Example: Calculate the time required for three-fourths of a sample of cesium-138 to decay given that its half-life is 32.2 minutes.

Answer 1

After one half-life, half is gone. After the second half-life, half of that half is gone, leaving only 1/4 of the remaining amount. So 3/4 has decayed. Just add two half-lives.