Answer:

12:00 or 24:00 am/pm

Step-by-step explanation:

21:30 = 9:30

21:30 = 9:302½ hours = 2:30

21:30 = 9:302½ hours = 2:309:30 + 2:30

21:30 = 9:302½ hours = 2:309:30 + 2:30= 12:00 am/pm

or

2½ hours = 2:30

2½ hours = 2:3021:30 + 2:30

2½ hours = 2:3021:30 + 2:30= 24:00 am/pm