Given that A, B, and C are all multiples of 7, AB=21, BC=7, and C is a factor of 7, find the sum of the three numbers A, B, and C.
已知A、B、C都是7的倍数，A-B=21，B-C=7，C是7的因数，求A、B、C三个数的和．
<gadget id="calculator">7 + 7</gadget>
<output>14</output>

<gadget id="calculator">21 + 14</gadget>
<output>35</output>

<gadget id="calculator">35 + 14 + 7</gadget>
<output>56</output>

<result>56</result>