The wise old man is 75 years old this year, and the ages of his three grandsons are 17, 15 and 13 years old. How many years later, the ages of the three grandsons are equal to the age of the wise old man.
智慧老人今年75岁，3个孙子的年龄分别是17岁、15岁和13岁，多少年后3个孙子的年龄等于智慧老人的年龄．
<gadget id="calculator">17 + 15 + 13</gadget>
<output>45</output>

<gadget id="calculator">75 - 45</gadget>
<output>30</output>

<gadget id="calculator">3 - 1</gadget>
<output>2</output>

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

<result>15</result>