There is a kind of forage grass on the grassland, which doubles every day, and on the 10th day, it has covered 72 mu of pasture. Covering 36 acres is the first number of days.
草原上有一种牧草，每天增长1倍，长到第10天，已覆盖72亩牧场．覆盖36亩是第多少天．
<result>9</result>