A can do a piece of work alonein 8 days. B can do the same work alone in 21 days. If they work together for 3 days, then how much work is completed?
A = 8 days
B= 21 days
Total work = lcm (8,21)= 168 days
A's effiency = $$168\div8$$ =21 days
B's effiency = $$168\div21$$ =8 days
A+B =Â Â $$168\div29$$
 A+B's one day work =  $$29\div168$$
 A+B's 3 day work =  $$29\div168\div3$$
                 =  $$\frac{29}{56}$$
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