甲、乙两个工厂生产某种设备，产能均为x件/天.已知甲先开始生产，且4月3日结束后乙的累计产量和4月11日结束后甲的累计产量之和为30x件。已知4月6日结束后，甲和乙的累计产量之比为4:3,问甲是从哪一天开始生产的?

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