Xiao Fang has a total of 20 pieces of RMB with a face value of 5 yuan and 10 yuan, totaling 180 yuan. Which face value is 10 yuan exactly how many sheets.
小方有面值为5元与10元的人民币共20张，合计180元．其中面值为10元正好有多少张．
<gadget id="calculator">10 * 20</gadget>
<output>200</output>

<gadget id="calculator">200 - 180</gadget>
<output>20</output>

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

<gadget id="calculator">20 / 5</gadget>
<output>4</output>

<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<result>16</result>