It takes 4 hours to process a batch of parts, and it takes 6 hours if the master and apprentice work together. After the two started working together for a period of time, the master left for other tasks, and the remaining tasks were completed by the apprentice alone, which took another 3 hours. How long did the master and apprentice start working together?
加工一批零件，师、徒二人合做要4小时完成，如果由师傅一人完成要6小时．二人开始合做一段时间后，师傅另有事情离开，余下的任务由徒弟单独来完成，又用了3小时．师、徒二人开始合做了几小时？
<gadget id="calculator">1 / 12</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">(1/12) * 3</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 / 4</gadget>
<output>1/4 = around 0.25</output>

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

<result>3</result>