The master and the apprentice have to complete the production task of 500 parts, the apprentice produces 15 parts per hour, and the master produces 20 parts per hour. After the apprentice produces 45 pieces first, the rest will be jointly produced by the master and the apprentice. How much time will it take to complete the production task?
师徒二人要完成500个零件的生产任务，徒弟每小时生产15个，师傅每小时生产20个．徒弟先生产45个后，剩下的由师徒二人合做，还要工作多少时间才能完成生产任务？
<gadget id="calculator">500 - 45</gadget>
<output>455</output>

<gadget id="calculator">15 + 20</gadget>
<output>35</output>

<gadget id="calculator">455 / 35</gadget>
<output>13</output>

<result>13</result>