Master Li processes a batch of parts. The ratio of processed and unprocessed parts is 1:3. After processing 400 more parts, the processed parts account for 1/3 of the total. How many parts are there in this batch?
李师傅加工一批零件，已加工与未的个数比是1：3，再加工400个后，已加工的占总数的(1/3)，这批零件有多少个？
<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">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 - (1/3)</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">(3/4) - (2/3)</gadget>
<output>1/12 = around 0.083333</output>

<gadget id="calculator">400 / (1/12)</gadget>
<output>4_800</output>

<result>4_800</result>