A piece of pasture, if horses and cows are allowed to graze, the grass will be eaten in 45 days; if horses and sheep are allowed to graze, the grass will be eaten in 60 days; if cattle and sheep are allowed to graze, the grass will be eaten in 90 days. Cows and sheep eat grass every day is equal to horses eat grass every day. Now let horses, cows, and sheep eat grass together. How many days can they eat a piece of grass?
一片牧草，如果让马和牛去吃草，45天将草吃完，如果让马和羊去吃草，60天将草吃完，如果让牛和羊去吃草，90天吃完，已知牛和羊每天吃草等于马每天吃草，现在让马，牛，羊一起去吃草多少天可以吃完一片草？
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