A project can be completed in 10 days by A alone, and 15 days by B alone. If two people cooperate, the efficiency of A will be reduced by 20%, and the efficiency of B will be reduced by 10%. If the project is completed in 9 days, both The number of days that two people cooperate should be as few as possible, so how many days should two people cooperate?
一项工程，甲单独做需要10天完成，乙单独做需要15天完成，如果两人合作，甲的效率要降低20%，乙的效率要降低10%，如果9天完成这项工程，两人合作的天数要尽可能少，那么两人要合作多少天？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/10) * 9</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">1 - (9/10)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">20 / 100</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">(1/10) * (4/5)</gadget>
<output>2/25 = around 0.08</output>

<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">10 / 100</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">1 - (1/10)</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">(1/15) * (9/10)</gadget>
<output>3/50 = around 0.06</output>

<gadget id="calculator">(3/50) - (1/10)</gadget>
<output>-1/25 = around -0.04</output>

<gadget id="calculator">(2/25) + (-1/25)</gadget>
<output>1/25 = around 0.04</output>

<gadget id="calculator">(1/10) / (1/25)</gadget>
<output>5/2 = around 2.5</output>

<result>5/2 = around 2.5</result>