A and B together can do a piece of work in 50 days. If A is 40 % less efficient than B, in how many days can A working alone complete 60 % of the work?

80

A+B= 50 days

As given A is 40% less efficient than B, (= \(\frac{2}{5}\)) less efficient than B therefore 

                 A  :  B

Efficiency   3  :  5

so,

Total work be (3+5) x 50 = 400        ..(Total efficiency × Days = Total work) 

60% of the work = 60% of 400 = 240 units

Time taken by A to complete 60% of total work = \(\frac{240}{3}\) = 80 days