The two masters and apprentices produce a batch of parts. The master works alone for 5 hours and the apprentice alone for 8 hours. After two hours of joint work, there are still 42 unfinished parts. How many parts are there in this batch?
师徒二人生产一批零件，师傅单独做5小时完成，徒弟单独做8小时完成，两人合做2小时后，还有42个没做完，这批零件有多少个？
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 / 8</gadget>
<output>1/8 = around 0.125</output>

<gadget id="calculator">(1/5) + (1/8)</gadget>
<output>13/40 = around 0.325</output>

<gadget id="calculator">(13/40) * 2</gadget>
<output>13/20 = around 0.65</output>

<gadget id="calculator">1 - (13/20)</gadget>
<output>7/20 = around 0.35</output>

<gadget id="calculator">42 / (7/20)</gadget>
<output>120</output>

<result>120</result>