The sum of the numbers A, B, and C is 830. The number A is 20 less than the number B, and the number C is 10 more than the number B. Find the number B.
甲、乙、丙三数之和是830，甲数比乙数少20，丙数比乙数多10，求乙数．
<gadget id="calculator">20 - 10</gadget>
<output>10</output>

<gadget id="calculator">830 + 10</gadget>
<output>840</output>

<gadget id="calculator">840 / 3</gadget>
<output>280</output>

<result>280</result>