Answer:
15a: 3 tarts = 90 minutes or 1 tart = 30 minutes
15b: 3.75 days
15c: 5 = 120 minutes or 1 tart every 24 minutes. It would take 720 minutes, or 12 hours, and if Peter is working 4 hrs a day, it would take 3 days.
Explanation:
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