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题库 / Prep2007E2-PS-102
A certain store found that last year the average (arithmetic mean) monthly sales from January through October was $120,000. In November and December, the monthly sales were 3 and 5 times, respectively, the average for the other 10 months. What was the store's average monthly sales last year?
$180,000
$216,000
$252,000
$318,000
$360,000
还没有题目讲解（毕出老师会陆续发布对官方考题的解读，请保持关注）。
X=(120000*10+3*120000+5*120000)/12
X=(1200000+360000+600000)/12
= 2160000/12
=180000
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