Both a and b are natural numbers not equal to 0, and (1/(a))+(1/(b))=(4/15), then a+b=what
a和b都是不等于0的自然数，且(1/(a))+(1/(b))=(4/15)，则a+b=多少
<gadget id="calculator">5 + 15</gadget>
<output>20</output>

<result>20</result>