A and B can do a work in 40 days. B and C together can complete the same work in 30 days, while A and C together can complete the same work in 20 days. How long (in days) will A alone take to complete 75% of the work?

36 days

Efficiency of ( A + B + C) = 13R

Efficiency of ( A + B + C)  = \(\frac{13R}{2}\)

Efficiency of A = \(\frac{13R}{2}\) - 4R = \(\frac{5R}{2}\)

Time taken by A to complete 75% of the work

                           = 120R × \(\frac{75}{100}\) × \(\frac{2}{5R}\) = 36 days