Xiao Qiang practiced running for 4 minutes (that is, running for 4 minutes). Due to the decline in physical strength, the length of the subsequent run in each minute was reduced by the same number of meters compared with the length of the previous minute. It is known that Xiao Qiang ran 500 meters in the first two minutes. If he ran 420 meters in two minutes, how many meters did he run in the first minute?
肖强练习4分钟跑(即跑4分钟)，由于体力下降，后面每分钟跑的长度都比前一分钟跑的长度减少相同的米数，已知肖强前面两分钟跑了500米，后面两分钟跑了420米，则他第一分钟跑了多少米？
<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">2 + 2</gadget>
<output>4</output>

<gadget id="calculator">80 / 4</gadget>
<output>20</output>

<gadget id="calculator">500 + 20</gadget>
<output>520</output>

<gadget id="calculator">520 / 2</gadget>
<output>260</output>

<result>260</result>