There is a project that A alone completes in a day, and B alone completes it in b days. Now A first completes the project in 4 days, and A and B cooperate to complete the rest in 3 days. What is the minimum value of a? (a and b are both positive integers)
有一项工程，甲单独做a天完成，乙单独做b天完成，现在由甲先做4天，余下的由甲、乙合作3天完成．a最小是多少？(a、b都是正整数)
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