There is a vegetable field and a wheat field. Half of the vegetable field is placed with the wheat field for a total of 81 hectares. Half of the wheat field is placed with the vegetable field for a total of 84 hectares. How many hectares is the vegetable field?
有一块菜地和一块麦地，菜地的一半和麦地放在一起共81公顷，麦地的一半和菜地放在一起共84公顷，菜地是多少公顷？
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">81 * (1/2)</gadget>
<output>81/2 = around 40.5</output>

<gadget id="calculator">84 - (81/2)</gadget>
<output>87/2 = around 43.5</output>

<gadget id="calculator">(1/2) * (1/2)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">(87/2) / (3/4)</gadget>
<output>58</output>

<result>58</result>