The number of A is 2784, which is 639 more than the number of B. What is the number of B? (List the equations containing the unknown χ, and then solve them)
甲数是2784，比乙数多639，乙数是多少？(列出含有未知数χ的等式，再解答出来)
<gadget id="calculator">2_748 - 639</gadget>
<output>2_109</output>

<result>2_109</result>