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In the past, one discount coupons can only use once, and the merchants will throw away the old and issues the new coupons. But nowadays, there are six discount circles in one coupon, and each circle stands for one discount coupon. That means, one coupon can be used for six times now, it save the resources a lot.
zhengzhou University of light
ZHENGZHOU, China
DANMING WANG,
ZHENZHOU University of light