33. Happy Farm has two sheep pens, one in the east and one in the west. The eastern pen has 42 sheep, and the western pen has 66 sheep. Every day, 4 sheep move from the eastern pen to the western pen, and 7 sheep move from the western pen to the eastern pen. After $\qquad$ days, the number of sheep in the eastern pen will be 5 times the number of sheep in the western pen.
$16$