Question 73763
Let t be the time that Jeff drives. Then the time that Lauren drives is t + 1/2 hour.
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The basic equation we will be using is Distance = Rate * Time
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The distance Lauren covers has a rate of 70 mph and a time of (t + 1/2) so her distance is
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D = r*t = 70*(t + 1/2) = 70*t + 35
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Jeff drives for time t at a rate of 75 mph. So Jeff's distance is:
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D = r*t = 75*t
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When the sum of these two distances equals 370 miles, Jeff and Lauren's cell phones will 
stop operating. So we can set up the equation:
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70*t + 35 + 75t = 370
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Adding the terms containing t results in:
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145*t + 35 = 370
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Eliminate the 35 on the left side by subtracting 35 from both sides to get:
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145*t = 370 - 35 = 335
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Solve this equation by dividing both sides by 145 to find that:
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t = 335/145 = 2.310 hours
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which is about 2 hours and 19 minutes