问题
单项选择题
If Mark saved an average (arithmetic mean) of $80 per week for 3 consecutive weeks, how much did he save the second week ？（）
(1)The average amount that Mark saved per week for the first 2 weeks was $60.
(2)The amount that Mark saved the first week was 
the amount he saved the second week and 
the amount he saved the third Week.
A．条件(1)充分，但条件(2)不充分．
B．条件(2)充分，但条件(1)不充分．
C．条件(1)和(2)单独都不充分，但条件(1)和条件(2)联合起来充分．
D．条件(1)充分，条件(2)也充分．
E．条件(1)和条件(2)单独都不充分，条件(1)和条件(2)联合起来也不充分．
答案
参考答案：B
解析：
(1)中指Mark在前两周中平均每周存60美元，但无法确定第一、二周分别存多少钱；(2)中指出第一周存的钱等于第二周存的钱的[*]，等于第三周存的钱的[*]，设第一周存的钱为x，由题意可得到下面的式子：
[*]
也即Mark第二周存了$80。