How is this solved step by step

The School's maintenance department has 4 painters. If one painter goes to an office to do the painting, it takes her 10 minutes to get set up to paint, followed by 10 minutes to paint each wall (50 minutes total to paint all 4 walls). Due to the disruption caused by a painter, each office must be vacated for the full time it is being painted (but no other office need be vacated). If all 4 employees go, then it takes each employee the same 10 minute setup but the 4 walls then take only another 10 minutes to be painted simultaneously (the office must be vacated for only 20 minutes). While the School wants to vacate each person's office for the shortest time possible, it must get 4 offices painted per hour on average. What should the batch size be for the walls in an office?

I know the answer is Batch size of 2 (2 wails should be painted by one painter)