64 people subscribed to three kinds of magazines A, B, and C, 28 people subscribed to type A, 41 people subscribed to type B, 20 people subscribed to type C, 10 people subscribed to two types of magazines A and B, and 10 people subscribed to type B, There are 12 people who ordered two kinds of C, and 12 people who ordered two kinds of A and C. Then how many people are booked for all three.
64人订A、B、C三种杂志，订A种的有28人，订B种的有41人，订C种的有20人，订A、B两种的有10人，订B、C两种的有12人，订A、C两种的有12人．则三种都订的有多少人．
<gadget id="calculator">10 + 12 + 12</gadget>
<output>34</output>

<gadget id="calculator">20 - 64</gadget>
<output>-44</output>

<gadget id="calculator">28 + 41 + (-44)</gadget>
<output>25</output>

<gadget id="calculator">34 - 25</gadget>
<output>9</output>

<result>9</result>