Mom is going to buy 3 skirts and 4 tops for Xiaohong, and she should pay 680 yuan. Because she doesn't have enough money, she only buys 3 skirts and 1 top, and only pays 260 yuan. How much is a skirt?
妈妈准备给小红买3条裙子和4件上衣，应该付680元，因为带的钱不够，就只买了3条裙子和1件上衣，只付了260元．一条裙子多少元？
<gadget id="calculator">680 - 260</gadget>
<output>420</output>

<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">420 / 3</gadget>
<output>140</output>

<gadget id="calculator">260 - 140</gadget>
<output>120</output>

<gadget id="calculator">120 / 3</gadget>
<output>40</output>

<result>40</result>