a, b and c can do a piece of work in 7 days, 14 days and 28 days respectively. how long will they take to finish the work, if all the three work together?	Select:
A) 4	B) 9	C) 2	D) 11	E) none
<gadget id="calculator">1 / 28</gadget>
<output>1/28 = around 0.035714</output>

<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">1 / 14</gadget>
<output>1/14 = around 0.071429</output>

<gadget id="calculator">(1/7) + (1/14)</gadget>
<output>3/14 = around 0.214286</output>

<gadget id="calculator">(1/28) + (3/14)</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / (1/4)</gadget>
<output>4</output>

<result>A</result>
a, b and c can do a piece of work in 7 days, 14 days and 28 days respectively. how long will they take to finish the work, if all the three work together?
inverse(add(inverse(28), add(inverse(7), inverse(14))))
"1 / 7 + 1 / 14 + 1 / 28 = 7 / 28 = 1 / 4 all three can finish the work in 4 days answer : a"