A is 20% more efficient than B and C is 25% less efficient than B. Working together A, B and C can complete the work in 20 days. How many days will A need to do 80% of the work?

49 \(\frac{3}{5}\) days

Ratio of efficiency of    A   :   B    :  C

                               100  :  120                                          

                                          100 : 75

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                                  10 :  12   :  9

Total work = efficiency x no. of days 

                = 31 x 20 = 620

Time taken by A to complete 80% of the work = \(\frac{80}{100}\)  x \(\frac{Total\; work}{Efficiency}\)

                                                                   = \(\frac{4}{5}\)  x \(\frac{620}{10}\) = \(\frac{248}{5}\) = 49 \(\frac{3}{5}\) days