SOLUTION: Please help me with this problem: A jogger is training for a marathon. She wants to run 20 miles in two hours. She runs the first 10 miles at 8 mph. How fast must she run the la

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Question 913530: Please help me with this problem:
A jogger is training for a marathon. She wants to run 20 miles in two hours. She runs the first 10 miles at 8 mph. How fast must she run the last 10 miles to achieve her goal?
Found 2 solutions by jim_thompson5910, josgarithmetic:
Answer by jim_thompson5910(35256) (Show Source):
You can put this solution on YOUR website!
"She runs the first 10 miles at 8 mph" so she spent 10/8 = 5/4 = 1.25 hours so far running.

She has 2 - 1.25 = 0.75 hours leftover.

She has 10 more miles left to run.

-------------------------------------------------------

Distance = Rate * Time

D = r*t

10 = r*0.75

10/0.75 = r

13.3333 = r

r = 13.3333

She has to run approximately 13.3333 mph (on average) to achieve her goal.

Let me know if you need more help or if you need me to explain a step in more detail.
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Thanks,

Jim

Answer by josgarithmetic(39630) (Show Source):
You can put this solution on YOUR website!
She wants to run 20 miles in 2 hours OR an average of 10 miles per hour.

RT=D
Let r be the speed during the second ten miles.

and

First equation describes the first ten miles, and the second equation describes the last ten miles. Solve for t in the first equation, and use it in the second equation, and then solve the second equation for r.