Adam completes 60% of a task in 15 days and then takes the help of Bob

Given: Adam completes 60% of a task in 15 days and then takes the help of Bob and Carol. Bob is 50% as efficient as Adam is and Carol is 50% as efficient as Bob is.

Asked: In how many more days will they complete the task?

Balance work = 40%
Adam completes 60% of a task in 15 days
Adam complete 100% of the task in 15*100/60 = 25 days

Bob is 50% as efficient as Adam is
Bob completes 100% of the task in 50 days.

and Carol is 50% as efficient as Bob is.
Carol completes 100% of the task in 100 days.

In how many more days will they complete the task =\(40\% / (\frac{1}{25} + \frac{1}{50} + \frac{1}{100}) = \frac{2}{5} * \frac{100}{7} = \frac{40}{7} = 5 \frac{5}{7}\) days

IMO C