At the end of his first week, Brian now has doubled his initial time of 10 seconds to 10 * 2 = 20 seconds. At the end of his second week, Brian doubled the previous week's time of 20 seconds to 20 * 2 = 40 seconds. At the end of his third week of practice, Brian adds another 50% of 40 seconds to his time so 40 * .5 = 20 seconds added. Having added 20 seconds to his time from his second week, Brian can now hold his breath for 40 + 20 = 60 seconds.
This justifies what answer for what question? Q & A:
Brian can only hold his breath underwater for 10 seconds. He wants to get better, so he starts practicing. After a week, he's doubled the amount of time he can do it. After another week, he's doubled it again from the previous week. The final week, he's increased it by 50% from the previous week. How long can Brian hold his breath for now?
60