A mother planted a 16-inch tree on her son's first birthday.  On the day the tree was planted, the boy was 24 inches tall.  If the tree grows twice as fast as the boy does, how tall will the tree be by the time the boy is 36 inches tall?
The boy grew 36 - 24 = <<36-24=12>>12 inches since the tree was planted.
Since the tree grows twice as fast as the boy, it will have grown 12*2 = <<12*2=24>>24 inches during this time period.
Thus, the final height of the tree is 16 + 24 = <<16+24=40>>40 inches.
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