Hi Jack, You must hand in the order list of books by the first working day of every month. The rules for choosing books are： For textbooks： ※ August is coming. We must have all textbooks, without thinking of other rules. For books which are not textbooks： ※ If more than 5 copies were sold last month, we choose them. But if there are 5 or more in stock(现货)，we do not make an order. ※ If the wholesale(批发) prices are higher than ﹩75, we don't choose them. And we don't order anything whose retail(零售) price is higher than ﹩100. | 
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，则实数a，b应满足的条件是