Tian Tian asked Teacher Zhang how old he was, and Teacher Zhang said: "Add 9 to my age, divide by 4, subtract 15, and multiply by 10, it is exactly 20 years old." How old is Teacher Zhang.
甜甜问张老师多大年纪，张老师说："把我的年纪加上9，除以4，减去15，再乘10，恰好是20岁．"张老师多少岁．
<gadget id="calculator">20 / 10</gadget>
<output>2</output>

<gadget id="calculator">2 + 15</gadget>
<output>17</output>

<gadget id="calculator">17 * 4</gadget>
<output>68</output>

<gadget id="calculator">68 - 9</gadget>
<output>59</output>

<result>59</result>