The students have to clean up after the party.
How long had they been cleaning up before another people came?
Example: Patrick came at 8:40. When Patrick came there, the students had been taking out the litter bins for 10 minutes.
|8:00 - 8:30||assign the tasks|
|8:30 - 9:00||take out the litter bins|
|10:00 - 11:00||pick up the rubbish|
|11:00 - 12:00||do the dishes|
|12:00 - 13:00||vacuum the carpets|