A travels 9 kilometers per hour, and B travels 3 kilometers less than A per hour. The two travel from two places 20 kilometers apart at the same time, and after a few hours, the distance between the two is 80 kilometers.
甲每小时行9千米，乙每小时比甲少行3千米，两人从相隔20千米的两地同时相背而行，多少小时后两人相隔80千米．
<gadget id="calculator">80 - 20</gadget>
<output>60</output>

<gadget id="calculator">9 - 3</gadget>
<output>6</output>

<gadget id="calculator">6 + 9</gadget>
<output>15</output>

<gadget id="calculator">60 / 15</gadget>
<output>4</output>

<result>4</result>