The agricultural machinery store sells a batch of new agricultural tractors, half of the total number is sold in the morning, more than 20 units, and the remaining half is sold in the afternoon, more than 15 units. At this time, there are only 75 units left. How many farm tractors are there in this batch?
农机店出售一批新型的农用拖拉机，上午售出总数的一半多20台，下午售出剩下的一半多15台，这时只剩75台．这批农用拖拉机共有多少台？
<gadget id="calculator">75 + 15</gadget>
<output>90</output>

<gadget id="calculator">90 * 2</gadget>
<output>180</output>

<gadget id="calculator">180 + 20</gadget>
<output>200</output>

<gadget id="calculator">200 * 2</gadget>
<output>400</output>

<result>400</result>