When Suguo Supermarket went to Nanjing to buy goods, it traveled an average of 90 kilometers per hour when it went there; after purchasing the goods, it traveled an average of 60 kilometers per hour when it returned to the original road. How many kilometers per hour is the average purchase this time?
苏果超市到南京进货，去时平均每小时行90千米；进货后原路返回平均每小时行60千米．这次进货平均每小时行多少千米．
<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">1 / 90</gadget>
<output>1/90 = around 0.011111</output>

<gadget id="calculator">1 / 60</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">(1/90) + (1/60)</gadget>
<output>1/36 = around 0.027778</output>

<gadget id="calculator">2 / (1/36)</gadget>
<output>72</output>

<result>72</result>