Answer:
21.00 hours
Explanation:
4 hours, 30 minutes=4.5 hours
4 hours, 45 minutes=4.75 hours
5 hours, 30 minutes=5.5 hours
6 hours, 15 minutes=6.25 hours
4.5 + 4.75 + 5.5 + 6.25 = 21.00 hours
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