Processing a batch of parts, if processing 35 parts per hour, can be completed 1 hour earlier than the original plan; if processing 42 parts per hour, it can be completed 4 hours earlier than the original plan. How many parts are there in total?
加工一批零件，如果每小时加工35个，可比原计划提前1小时完成；如果每小时加工42个，可比原计划提前4小时完成．求这批零件共有多少个？
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">1 / 35</gadget>
<output>1/35 = around 0.028571</output>

<gadget id="calculator">1 / 42</gadget>
<output>1/42 = around 0.02381</output>

<gadget id="calculator">(1/35) - (1/42)</gadget>
<output>1/210 = around 0.004762</output>

<gadget id="calculator">3 / (1/210)</gadget>
<output>630</output>

<result>630</result>