The father is 49 years old this year, and the son is 21 years old this year. How many years ago, the father was 5 times the age of the son.
父亲今年49岁，儿子今年21岁，多少年前父亲的年龄是儿子的5倍．
<gadget id="calculator">21 * 5</gadget>
<output>105</output>

<gadget id="calculator">105 - 49</gadget>
<output>56</output>

<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">56 / 4</gadget>
<output>14</output>

<result>14</result>