For paving pavement floor tiles on Guoguo Road, engineering team A alone can complete the paving in 20 days, and engineering team B can complete this work in 9 days (3/4). How many days can it be done if the two teams work together?
虢国路铺人行道地砖，甲工程队单独铺20天可以完成，乙工程队9天可以完成这项工作的(3/4)．如果两队合作，多少天可以完成？
<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">(3/4) / 9</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/20) + (1/12)</gadget>
<output>2/15 = around 0.133333</output>

<gadget id="calculator">1 / (2/15)</gadget>
<output>15/2 = around 7.5</output>

<result>15/2 = around 7.5</result>