If both m and n are non-zero natural numbers and satisfy ((m)/3)+((n)/7)=(16/21), then m+n=how much.
如果m和n都是非零的自然数，并且满足((m)/3)+((n)/7)=(16/21)，那么m+n=多少．
<gadget id="calculator">1 + 3</gadget>
<output>4</output>

<result>4</result>