单选题
If 12a + 3b = 1 and 7b - 2a = 9, what is the average (arithmetic mean) of a and b ?
A. 0.1
B. 0.5
C. 1
D. 2.5
E. 5
A
B
C
D
E
【正确答案】
B
【答案解析】
Add the two equations: [*] Do not waste time solving for a and b.
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