国王两只手里都抓了一把金币，一把是单数，另一把是双数，让阿诺德猜猜哪只手里的金币是单数，猜中了国王就把些金币都给阿诺德。

阿诺德说：“您把右手里的金币数乘3，左手里的金币数乘2，然后把两次乘得的积相加的和告诉我，我就一定能猜出。”

国王算了一下，是43。阿诺德说：“您右手里的金币是单数。”国王只好把金币都奖励给了阿诺德。你知道阿诺德是怎样猜出的吗？

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