Use a piece of iron wire with a length of 7.9 decimeters to make a circular iron ring. The joint is 0.5 cm. What is the radius of this iron ring in cm?
用一根长7.9分米的铁丝做一个圆形铁圈，接头处是0.5厘米，这个铁圈的半径是多少厘米？
<gadget id="calculator">7.9 * 10</gadget>
<output>79</output>

<gadget id="calculator">79 - 0.5</gadget>
<output>78.5</output>

<gadget id="calculator">78.5 / 3.14 / 2</gadget>
<output>12.5</output>

<result>12.5</result>