To process a batch of parts, it is known that the ratio of the number of processed parts to the number of unprocessed parts is 1:3, and then process 18 parts, the ratio of the number of processed parts to the number of unprocessed parts is 2:3, How many parts are there in total?
加工一批零件，已知加工的个数与未加工的个数的比是1：3，再加工18个零件，已加工的零件个数与未加工的零件个数之比为2：3，则这批零件一共有多少个？
<gadget id="calculator">2 + 3</gadget>
<output>5</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

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

<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">(2/5) - (1/4)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">18 / (3/20)</gadget>
<output>120</output>

<result>120</result>