问题
单项选择题
If the average (arithmetic mean) of x and y is 40, and z-x=60, what is the average of y and z ？（）
A. 20
B. 50
C. 65
D. 70
E. 140
答案
参考答案：D
解析：
To get the average of y and z, you need to find half of the sum of y+ z. To obtain that number, first restate the first statement as an equation: x+y=80. Add that equation to z-x=60, so that x+y+z-x=80+60=y+z=140. The average of y and z is 140/2=70.
