It takes 15 hours for typists A and B to type a manuscript together. If A alone typed for 4 hours and B alone for 3 hours, they will jointly complete 25% of the manuscript. Ask: How many hours does each typist need to type this manuscript?
甲、乙两打字员合打一份稿件需要15小时，如果甲单独打4小时，乙单独打3小时，则共同完成这份稿件的25%．求：每个打字员单独打这份稿件各需要多少小时？
<gadget id="calculator">1 / 15</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/15) - (1/20)</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">1 / (1/60)</gadget>
<output>60</output>

<result>60</result>