Li Jiang takes a taxi from A to B. The starting fare is 15 yuan (within 4 kilometers), and 1 yuan for every 500 meters beyond 4 kilometers. When he reaches the end, he pays a total of 21 yuan for the taxi from A to B. How many kilometers away are the places?
李江乘出租车从甲地到乙地，起步价15元(4千米之内)，超过4千米每500米为1元，到达终点时，他共付车费21元，甲地到乙地相距多少千米？
<gadget id="calculator">21 - 15</gadget>
<output>6</output>

<gadget id="calculator">500 / 1_000</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">6 * (1/2)</gadget>
<output>3</output>

<gadget id="calculator">4 + 3</gadget>
<output>7</output>

<result>7</result>