There are 81 red, yellow and white balls, 5 less red balls than yellow balls, 4 more white balls than red balls, how many yellow balls are there?
有红、黄、白三种球81个，红球比黄球少5个，白球比红球多4个，黄球有多少个．
<gadget id="calculator">81 - 5 - 4</gadget>
<output>72</output>

<gadget id="calculator">1 + 1 + 1</gadget>
<output>3</output>

<gadget id="calculator">72 / 3</gadget>
<output>24</output>

<gadget id="calculator">24 + 5</gadget>
<output>29</output>

<result>29</result>