The grandfather is 78 years old this year, and the ages of the three grandchildren are 21, 17, and 10 years old. How many years have passed, and the age of the grandfather is equal to the sum of the ages of the three grandchildren.
今年爷爷78岁，三个孙子年龄分别为21岁、17岁、10岁，经过多少年，爷爷的年龄等于三个孙子的年龄和．
<gadget id="calculator">78 - 21 - 17 - 10</gadget>
<output>30</output>

<gadget id="calculator">1 - 1</gadget>
<output>0</output>

<gadget id="calculator">1 + 1 + 0</gadget>
<output>2</output>

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<result>15</result>