Xiao Ming has 10 banknotes of two yuan and five yuan, totaling 29 yuan. How many two-dollar banknotes are there?
小明有二元和五元纸币共10张，共29元．二元纸币有多少张．
<gadget id="calculator">10 * 5</gadget>
<output>50</output>

<gadget id="calculator">50 - 29</gadget>
<output>21</output>

<gadget id="calculator">5 - 2</gadget>
<output>3</output>

<gadget id="calculator">21 / 3</gadget>
<output>7</output>

<result>7</result>