Sheriff Black Cat received the alarm and moved hamsters to commit crimes in the supermarket. When the police car arrived at the supermarket, the moving hamster had fled to the city gate one minute ago. The black cat sheriff immediately drove after him, but the speed of the stolen sports car driven by the hamster was 30 kilometers per hour faster than the speed of the police car driven by the black cat sheriff, so the sports car arrived at the city gate 4 minutes earlier than the police car, and when the police car When arriving at the city gate, the sports car driven by the moving hamster has fled to 12 kilometers outside the city gate. Find the distance from the supermarket to the city gate.
黑猫警长接到报警，搬仓鼠在超市作案．当警车赶到超市时，搬仓鼠已于1分钟前逃向城门．黑猫警长立即驾车追赶，但搬仓鼠驾驶的偷来的跑车的速度比黑猫警长驾驶的警车的速度每小时快30千米，因此跑车比警车早4分钟到达城门，而当警车到达城门时，搬仓鼠驾驶的跑车已逃到城门外12千米处，求超市到城门的距离．
<gadget id="calculator">12 / 4</gadget>
<output>3</output>

<gadget id="calculator">30 / 60</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">3 - (1/2)</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">3 - (5/2)</gadget>
<output>1/2 = around 0.5</output>

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

<gadget id="calculator">3 * 6</gadget>
<output>18</output>

<gadget id="calculator">18 - 3</gadget>
<output>15</output>

<gadget id="calculator">3 * 15</gadget>
<output>45</output>

<result>45</result>