The grandfather is 75 years old this year, and the ages of the three grandsons are 17, 15, and 13 years old. How many years later, the sum of the ages of the three grandsons is equal to the age of the grandfather.
祖父今年75岁，3个孙子的年龄分别是17岁、15岁和13岁，多少年后3个孙子的年龄和等于祖父的年龄．
<gadget id="calculator">17 + 15 + 13</gadget>
<output>45</output>

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

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

<result>15</result>