Four children, A, B, C, and D, have a total of 81 books. If we add 2 to A, subtract 2 from B, multiply C by 2, and divide D by 2, the four children have the same number of books. How many copies are there?
甲、乙、丙、丁四位小朋友共有81本书，如果把每人的本数：甲加2、乙减2、丙乘以2、丁除以2后，四人所有的本数相等，甲原来有多少本？
<gadget id="calculator">81 - 2</gadget>
<output>79</output>

<gadget id="calculator">2 + 79</gadget>
<output>81</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 + 1 + (1/2) + 2</gadget>
<output>9/2 = around 4.5</output>

<gadget id="calculator">81 / (9/2)</gadget>
<output>18</output>

<gadget id="calculator">18 - 2</gadget>
<output>16</output>

<result>16</result>