The sum of the ages of A, B, and C is 30 years, and the age of B is half of the sum of the ages of A and C. How old is B?
甲、乙、丙三人的年龄和为30岁，乙的年龄是甲、丙年龄和的一半．乙多少岁．
<gadget id="calculator">2 + 1</gadget>
<output>3</output>

<gadget id="calculator">30 / 3</gadget>
<output>10</output>

<result>10</result>