The number A is 99, which is 21 more than three times the number B. What is the number B?
甲数是99，比乙数的3倍多21，乙数是多少．
<gadget id="calculator">99 - 21</gadget>
<output>78</output>

<gadget id="calculator">78 / 3</gadget>
<output>26</output>

<result>26</result>