There is a piece of land with a total of 157 mu. After plowing with two large and small tractors for 3.5 hours, 17 mu remains uncultivated. It is known that each small tractor plows 8 mu per hour. Ask how much more each large tractor plows per hour than the small tractor Acres of land.
有一块地共157亩，用大小拖拉机各两台耕了3.5小时后，还剩17亩未耕，已知每台小拖拉机每小时耕地8亩，问每台大拖拉机每小时比小拖拉机多耕多少亩地．
<gadget id="calculator">8 * 2 * 3.5</gadget>
<output>56</output>

<gadget id="calculator">157 - 17 - 56</gadget>
<output>84</output>

<gadget id="calculator">84 / 3.5 / 2</gadget>
<output>12</output>

<gadget id="calculator">12 - 8</gadget>
<output>4</output>

<result>4</result>