If A is thrice as fast as B and together can do a work in 21 days. In how many days A alone can do the work?
Select
A) 36 B) 42 C) 28 D) 54 E) 45
A’s one day’s work= 1/X   B’s one day’s work= 1/3x   A + B’s one day’s work= 1/x + 1/3x = 1/21   = 3+1/3x = 4/3x = 1/21   x = 21*4/3= 
<gadget id="calculator">21*4/3</gadget>
<output>28</output>
28   ANSWER:28
<result>C</result>
If A is thrice as fast as B and together can do a work in 21 days. In how many days A alone can do the work?