To type a manuscript, it takes 10 hours for A to type alone, and 15 hours for B to type alone. If A typed 4 hours first, and the rest is typed by B alone, how many hours will B need to type to complete?
打一份稿件，由甲单独打需要10小时，由乙单独打需要15小时，如果甲先打4小时，余下的在由乙单独打，乙还要打几个小时才能完成？
<gadget id="calculator">1 / 10</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/10) * 4</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (2/5)</gadget>
<output>3/5 = around 0.6</output>

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

<gadget id="calculator">(3/5) / (1/15)</gadget>
<output>9</output>

<result>9</result>