Q: Kris is blowing u balloons for her sister's party. She has 30 minutes to blow up as many balloons as possible and decides to enlist her brother's help to increase the number of balloons. Kris can blow up a total of 2 balloon per minute, and her brother works twice as fast. After 15 minutes, her brother doubles his speed and works at this new speed for the remaining 15 minutes. After the 30 minutes, how many balloons, in total, did Kris and her brother blow up?
A: 240
Explain how we arrive at this answer:
Explanation: Throughout the 30 minutes, Kris blows up 2 balloons per minute * 30 minutes = 60 balloons. Her brother works at double Kris' speed in the first 15 minutes so he works at 2 balloons per minute * 2 = 4 balloons per minute. So in the first 15 minutes, he blows up 4 balloons per minute * 15 minutes = 60 balloons. In the second 15 minutes, he is working at double the rate he did previously, so he blows up 4 balloons per minute * 2 = 8 balloons per minute. In the second 15 minutes, Kris' brother therefore blows up 8 balloons per minute * 15 minutes = 120 balloons. Throughout the 30 minutes, he has blown up a total of 60 + 120 = 180 balloons. Kris and her brother therefore blew up a total of 60 balloons from Kris + 180 balloons from Kris' brother = 240 balloons.