Answer:
3 hours
Step-by-step explanation:
After one hour, car a will be only 20 miles ahead of car b, after two hours, car a will only be 10 miles ahead of car b, and after 3 hour, they will both be equal. This is because car b is going 10 mph faster than car a, because 70-60=10. every hour, car b will catch up to car a by 10 miles.
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