It is known that ((a)/3) and ((b)/7) are both true fractions, and ((a)/3)+((b)/7)=(16/21), then a+b= How many.
已知((a)/3)和((b)/7)都是真分数，且((a)/3)+((b)/7)=(16/21)，则a+b=多少．
<gadget id="calculator">16 - 7</gadget>
<output>9</output>

<gadget id="calculator">9 / 3</gadget>
<output>3</output>

<gadget id="calculator">1 + 3</gadget>
<output>4</output>

<result>4</result>