An intern can do a job in 15 days. The manager and senior manager are busy with other priorities and thus take 25 and 40 days respectively to complete the task. How long will they take to finish the task if all of them work together

Working together, their one day’s work is given as:

$$\frac{1}{15}+\frac{1}{25}+\frac{1}{40}=\frac{79}{600}$$

Hence, they wil finish their work in
$$7\frac{47}{79}\,{days}$$