Let J be the amount of time Julie paints
G for Glene
and
N for Nicolai
They paint a total of 25 hours.
So we can expressed this total hours by :

Since Julie paints for 6 hours, J = 6
Glene for 8 hours, G = 8
Nicolai will be :

To get the percentage of each person, divide their time to the total hours of work multiplied by 100 :

Answers :
For Nicolai :

For Julie :

For Glene :

To check if the answers are correct, the sum of the percentages MUST BE equal to 100.
Checking :

Therefore, the answers are correct.