Xiaobing is 9 years old this year, and his grandmother is 60 years old this year. How many years later, his grandmother's age is exactly four times that of Xiaobing.
小兵今年9岁，奶奶今年60岁，多少年后奶奶的年龄正好是小兵的4倍．
<gadget id="calculator">9 * 4</gadget>
<output>36</output>

<gadget id="calculator">60 - 36</gadget>
<output>24</output>

<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">24 / 3</gadget>
<output>8</output>

<result>8</result>