51.3-□+19.2=6.72, □=how much.
51.3-□+19.2=6.72，□=多少．
<gadget id="calculator">51.3 + 19.2</gadget>
<output>70.5</output>

<gadget id="calculator">70.5 - 6.72</gadget>
<output>63.78</output>

<result>63.78</result>