So how the question describe is that Michael works on the model for awhile and had Albert coming to help out.


First you have to think about
1. What is the speed Michael and Albert works?
2. how long Michael worked on the model alone as stated in the question and how much of the model would have been completed when he was working alone.

With the leftover amount of model, it would be done by the efforts of both Micheal and Albert. So if they work together, the speed should be faster and they would work together.

By figuring out the speed that they worked together, you can find out how Long they worked together too :)