Here are some possible mathematical problems that could be posed from the given situation:
1) The master worker can paint 1 square meter of the billboard per hour. The apprentice can paint 0.5 square meters per hour. If they work for 8 hours, how many square meters of the billboard can they paint?
2) The billboard measures 20 square meters. If the master worker and apprentice work together for x hours, write an equation to represent the relationship between the number of hours worked (x) and the area painted (y).
3) The billboard measures 20 square meters. The master worker and apprentice work together for 8 hours. How much of the billboard do they paint?