Explanation: In 30 minutes, the reaction is 50% complete. As a result, t1/2 = 30 minutes.
In two half-lives, 75% of the process is accomplished. As a result, t = 2 ×30 = 60 minutes.
In three half-lives, 87.5 percent of the reaction is finished.
Hence, t = 3 × 30 = 90 minutes.