Some animals live on a grassland. The amount of new grass that grows each day is constant. If there are $20$ horses, they can eat all grass in $12$ days. If there are $60$ sheep, they can eat all grass in $24$ days. The amount of grass that a horse eats per day is equal to that $4$ sheep eat per day. $12$ horses and $88$ sheep can eat all grass on the grassland in~\uline{~~~~~~~~~~}~days. 
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