BeCareful!: Group Work

One of the professors in Math department gives a lot of homework. We have 3 particular students in this class called Alice, Ben, and Clark. When Alice and Ben works together they finish a day's homework in 5 hours. Clark and Alice together finish it in 3 hours as they kind of fancy each other :) and like to work together. Ben and Clark, together, finish in 4 hours. One day both Ben and Clark are sick and they cannot work. Alice has to do it alone. How long will it take(in hours)? Round to 2 decimal places.