Master Wang makes 70 parts per hour, and Master Li makes 50 parts per hour. The two start working at the same time and cooperate to make 840 parts. How many hours will it take to complete?
王师傅每小时做70个零件，李师傅每小时做50个零件，两人同时开工合做840个零件，经过几小时可完成？
<gadget id="calculator">70 + 50</gadget>
<output>120</output>

<gadget id="calculator">840 / 120</gadget>
<output>7</output>

<result>7</result>