A is 15 years older than B. Five years ago, A was twice as old as B. What is B's current age?
甲比乙大15岁，五年前甲年龄是乙年龄的两倍，求乙现在的年龄？
<gadget id="calculator">2 - 1</gadget>
<output>1</output>

<gadget id="calculator">15 / 1</gadget>
<output>15</output>

<gadget id="calculator">15 + 5</gadget>
<output>20</output>

<result>20</result>