The price of the teapot is 50 yuan, the cost is 30 yuan, and the cost of the teacup is 80% of the price. Now the store sells a teapot and 2 teacups as a set, and sells them at a 10% discount on the sum of their prices. In this way, a profit of 18 yuan can be obtained from selling a set. How much does a teacup cost?
茶壶定价50元，成本是30元，茶杯的成本是定价的80%．现在商店将一把茶壶与2只茶杯配套出售，并且按它们定价之和的九折出售，这样出售一套可获得利润18元．一只茶杯的成本是多少元？
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">18 - 45</gadget>
<output>-27</output>

<gadget id="calculator">30 + (-27)</gadget>
<output>3</output>

<gadget id="calculator">1.8 - 1.6</gadget>
<output>0.2</output>

<gadget id="calculator">3 / 0.2</gadget>
<output>15</output>

<gadget id="calculator">(4/5) * 15</gadget>
<output>12</output>

<result>12</result>