The circumference of the outer circle of a ring is 62.8cm, and the circumference of the inner circle is 15.7cm. Find the width of the ring.
一个圆环的外圆的周长是62.8cm，内圆的周长是15.7cm，求这个圆环的宽度．
<gadget id="calculator">62.8 / 3.14 / 2</gadget>
<output>10</output>

<gadget id="calculator">15.7 / 3.14 / 2</gadget>
<output>2.5</output>

<gadget id="calculator">10 - 2.5</gadget>
<output>7.5</output>

<result>7.5</result>