There are 9 coins, including 1 yuan, 5 yuan, 10 yuan and 50 yuan, and there is at least one coin of each kind. If the total value of these 9 coins is 177 yuan, how many 10 yuan coins must there be?
现有9个硬币，为1元，5元，10元以及50元共四种，且每种硬币至少有一个．若这9个硬币总值177元，则10元硬币必须有多少个．
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