The roses in the flower shop are 15 yuan a piece. Now buy 4 and get 2 free. Buy 6 at a time. How much is each cheaper?
花店的玫瑰花15元一支，现在买4支送2支，一次买6支，每支便宜多少钱？
<gadget id="calculator">4 + 2</gadget>
<output>6</output>

<gadget id="calculator">4 / 6</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">15 * (2/3)</gadget>
<output>10</output>

<gadget id="calculator">15 - 10</gadget>
<output>5</output>

<result>5</result>