Part (a)
distance = rate*time
rate = distance/time
rate = 84/4
rate = 21
Answer: 21 mph
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Part (b)
distance = rate*time
distance = 21*7
distance = 147
Answer: 147 miles
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Part (c)
distance = rate*time
time = distance/rate
time = 57/21
time = 2.71428571428571
time = 2.714
Answer: Approximately 2.714 hours