Question. Jill spends time every day talking to her boyfriend on the phone. The first day of the week she spends 5 minutes on the phone with her boyfriend. Each of the following days she spends twice as much time talking on the phone as the day before. After the 5th day her parents take the phone away from her because she was on it too long. How much time did Jill spend talking to her boyfriend that week?
Answer. 155
What step-by-step reasoning justifies that answer?
Reasoning: Jill starts off spending 5 minutes on the phone, and the following day she spends double that 5 minutes for 5 * 2 = 10 minutes. The following day Jill spends double the previous day's total of 10 minutes on the phone, for 10 * 2 = 20 minutes. On the fourth day, Jill spends double the previous day's total of 20 minutes on the phone, for 20 * 2 = 40 minutes. On the fifth and final day, Jill spends double the previous day's total of 40 minutes on the phone, for 40 * 2 = 80 minutes. All told, Jill spends 5 + 10 + 20 + 40 + 80 = 155 minutes on the phone over 5 days.