A rowed a boat from upstream A to downstream to B, and B rowed upstream from downstream B at the same time. After 12 hours, the two met. At this time, A has traveled half of the whole journey and 9 kilometers. If A is in still water The speed of rowing in the water is 4 kilometers per hour, and the speed of B in still water is 5 kilometers per hour, so what is the speed of the current? (column equation solution)
甲从上游A地划船顺流下行到B地，乙同时从下游的B地划船逆流而上，经过12小时两人相遇，这时甲已行了全程的一半又9千米，如果甲在静水里划行的速度是每小时4千米，乙在静水里划行的速度是每小时5千米，那么水流的速度是多少？(列方程解答)
<gadget id="calculator">2 / 12</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">9 * (1/6)</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">(3/2) - 4</gadget>
<output>-5/2 = around -2.5</output>

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

<gadget id="calculator">1 + 1</gadget>
<output>2</output>

<gadget id="calculator">(5/2) / 2</gadget>
<output>5/4 = around 1.25</output>

<result>5/4 = around 1.25</result>