I am going to try a different example.
Say Mike does 1/6 job in an hours, then then Josh does 2/6 of a job in an hour which adds to 1/2 a job in an hour.
Since mike does 1/6 of a job in an hour, Kelvin must do 1/6 of a job in an hour too totalling 1/3 of a job in an hour.
So together they do 1/6 + 2/6 + 1/6 = 4/6 = 2/3 job in an hour = 3/2 hours for a job
So this time they take 1.5 hours to do the job which is different from my first answer.
So either my logic is up the creek or there is not just one definite answer.
I see CPhill is answering, he is better at these then me so we will see what he says :)