The ratio of A and B is 5:7, and the ratio of B and C is 3:4. It is known that the difference between A and B is 84. What is the sum of B and C?
甲、乙两数的比是5：7，乙、丙两数的比是3：4，已知甲、乙两数的差是84，求乙、丙两数的和是多少？
<gadget id="calculator">15 + 21 + 28</gadget>
<output>64</output>

<gadget id="calculator">21 / 64</gadget>
<output>21/64 = around 0.328125</output>

<gadget id="calculator">15 / 64</gadget>
<output>15/64 = around 0.234375</output>

<gadget id="calculator">(21/64) - (15/64)</gadget>
<output>3/32 = around 0.09375</output>

<gadget id="calculator">84 / (3/32)</gadget>
<output>896</output>

<gadget id="calculator">28 / 64</gadget>
<output>7/16 = around 0.4375</output>

<gadget id="calculator">(21/64) + (7/16)</gadget>
<output>49/64 = around 0.765625</output>

<gadget id="calculator">896 * (49/64)</gadget>
<output>686</output>

<result>686</result>