A and B can do a job in 10 days and 5 days, respectively. They worked together for two days, after which B was replaced by C and the work was finished in the next three days. How long will C alone take to finish 60% of the job?

18 days

A = 10 days, B = 5 days,

⇒ A + B worked for 2 days = (1 + 2) x 2 = 6 units    ..(Efficiency × Days = Total work)

⇒ Then, b is replaced by A + C and remaining work is completed in 3 days = \(\frac{4}{1 + C}\) = 3 days.

⇒ So, C is a day = \(\frac{1}{3}\)  (Efficiency)

⇒ Time required for C to complete 60% of total work i.e; 6 units = \(\frac{6}{1/3}\) = 18 days.         ..(\(\frac{Work}{Efficiency}\) = Time)