美国现在流通中的磁条信用卡和储值卡超过50亿张，我们可以用这个数字乘以3美元。
Multiply $3 by the more than 5 billion magstripe credit and prepaid cards in circulation in the U.S.
这是一种三位数字的代码，印在信用卡的背面，多数，但不是所有的电子零售商在进行买卖交易是都要求提供这一代码。
That's the three-digit code on the back of every credit card that most, but not all, e-retailers ask for when making a purchase.
他算出他同学信用卡的具体数字。
He figured out the exact figures of his classmates' credit CARDS.
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