Question 719654
You can ride a bike at 15 mph. Your sister rides 12 mph. You give her a one-hour head start. In how many hours will you catch up if neither of you stops.

(15 + x) (12 + x)
15x - 12x

In 2 hours
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In 1 hour, she's 12 miles away (12*1)
You "gain on her" at 3 mi/hr (15-12)
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t = d/r = 12/3 = 4 hours