For a manuscript, it takes 4 hours for A to type alone, and 5 hours for B to type alone. After A played for 1 hour, A and B finished the rest together. How many hours will it take to type this manuscript?
一份书稿，甲单独打需要4小时，乙单独打需要5小时．甲打了1小时后剩下的由甲、乙一起打完．这份稿件打完需要多少小时．
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">(1/4) + (1/5)</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">(3/4) / (9/20)</gadget>
<output>5/3 = around 1.666667</output>

<gadget id="calculator">(5/3) + 1</gadget>
<output>8/3 = around 2.666667</output>

<result>8/3 = around 2.666667</result>