For a manuscript, it takes 10 hours for A to type alone, and 15 hours for B to type alone. Now A will type for 4 hours alone, and then the two of them will type together. How many hours will it take?
一份稿件，甲单独打用10小时，乙单独打用15小时，现在先由甲单独打4小时，再两人合打，还需要多少小时？
<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">(1/10) + (1/15)</gadget>
<output>1/6 = around 0.166667</output>

<gadget id="calculator">(3/5) / (1/6)</gadget>
<output>18/5 = around 3.6</output>

<result>18/5 = around 3.6</result>