It takes 10 hours for a boat to go back and forth between two ports 180 kilometers apart, going down the river and 15 hours going up the river. Since the speed of the water increases after the heavy rain, it only takes 9 hours for the boat to travel along the current, how many hours does it take to travel against the current?
船往返于相距180千米的两港之间，顺水而下需用10小时，逆水而上需用15小时．由于暴雨后水速增加，该船顺水而行只需9小时，那么逆水而行需要多少小时．
<gadget id="calculator">180 / 10</gadget>
<output>18</output>

<gadget id="calculator">180 / 15</gadget>
<output>12</output>

<gadget id="calculator">18 + 12</gadget>
<output>30</output>

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<gadget id="calculator">180 / 9</gadget>
<output>20</output>

<gadget id="calculator">20 - 15</gadget>
<output>5</output>

<gadget id="calculator">15 - 5</gadget>
<output>10</output>

<result>18</result>