The main formula we use here is Work=Rate*Time,
1. The rate (speed of working) of Dominik is
2. Let the the rate of Katarina be
Together they have a rate of
equal to 
(project per hour)
so
simplify the units:
so the rate of Katarina is 3/40 (proj per hour)
Now, the rate of Dominik was 1/8 (pr/hr)=5/40 (pr/hr)
In 5 hours Dominik complets 5*5/40=25/40 of the project
Katarina completes 5*3/40=15/40 of the project.
1. The rate (speed of working) of Dominik is
2. Let the the rate of Katarina be
Together they have a rate of
so
simplify the units:
so the rate of Katarina is 3/40 (proj per hour)
Now, the rate of Dominik was 1/8 (pr/hr)=5/40 (pr/hr)
In 5 hours Dominik complets 5*5/40=25/40 of the project
Katarina completes 5*3/40=15/40 of the project.