There are two piles of coal. The ratio of the first pile to the second pile is 15:7. After being transported away from the first pile (1/5), the second pile is still 3.5 tons less than the first pile. How many tons of original coal?
有两堆煤，原来第一堆和第二堆的比是15：7，从第一堆运走(1/5)后，这时第二堆还比第一堆少3.5吨，第一堆原有煤多少吨？
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">15 * (4/5)</gadget>
<output>12</output>

<gadget id="calculator">12 - 7</gadget>
<output>5</output>

<gadget id="calculator">3.5 / 5</gadget>
<output>0.7</output>

<gadget id="calculator">0.7 * 15</gadget>
<output>10.5</output>

<result>10.5</result>