stationery store purchased 6400 copies of 4 kinds of exercise books for 16000 yuan. The unit price of each copy is: Type A 4 yuan, Type B 3 yuan, Type C 2 yuan, Type D 1.4 yuan. If the number of A and C is the same, and the number of B and D is also the same, how many copies of D are bought in total?
某文具店用16000元购进4种练习本共6400本．每本的单价是：甲种4元，乙种3元，丙种2元，丁种1.4元．如果甲、丙两种本数相同，乙、丁两种本数也相同，那么丁种练习本共买了多少本．
<gadget id="calculator">4 + 2</gadget>
<output>6</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<gadget id="calculator">6_400 * 3</gadget>
<output>19_200</output>

<gadget id="calculator">19_200 - 16_000</gadget>
<output>3_200</output>

<gadget id="calculator">3 + 1.4</gadget>
<output>4.4</output>

<gadget id="calculator">4.4 / 2</gadget>
<output>2.2</output>

<gadget id="calculator">3 - 2.2</gadget>
<output>0.8</output>

<gadget id="calculator">3_200 / 0.8 / 2</gadget>
<output>2_000</output>

<result>2_000</result>