SOLUTION: Max runs the first km of her training session at 8km/h. At least how many kilometers must she run at 15 km/h in order to average 12km/h for the whole session?

Question 100921: Max runs the first km of her training session at 8km/h. At least how many kilometers must she run at 15 km/h in order to average 12km/h for the whole session?
Answer by ankor@dixie-net.com(22740) (Show Source): You can put this solution on YOUR website!
Max runs the first km of her training session at 8km/h. At least how many kilometers must she run at 15 km/h in order to average 12km/h for the whole session
:
Time = distance/speed
:
Let d = distance required at 15 km/h to average 12km/h
:
Write a time equation
Time at 8 km/h + time at 15 km/h = time at 12/km/h
+ =
:
Multiply equation by 120 to get rid of the denominators
120* + 120* = 120*
:
Cancel out the denominators:
15(1) + 8(d) = 10(d+1)
:
15 + 8d = 10d + 10
:
8d - 10d = 10 - 15
:
-2d = -5
:
d = + 5/2 or 2.5 km at 15 km/hr rate
:
Total distance would be 3.5 km
:
Check solution by calculating av speed = dist/time
:
Find the time:
+ =
+ =
+ = 7/24 hr, is the total time
:
Check average speed (dist/time)
7/2 (3.5) divided by 7/24
* = 24/2 = 12 km/hr
:
Sorry, I got twisted around on the first attempt
RELATED QUESTIONS
Amber has entered a cross-country running series. Each race in the series has a time... (answered by mananth,MathTherapy)

Alice is in a 20-km race. She always runs the first half at an average speed of 2 km/h... (answered by ewatrrr)

Buduli covers a distance of 8km to his house in 1hour.He walks part of the distance at... (answered by ikleyn,MathTherapy)

Peggy drives to town at a rate of 40 km/h. How fast must she drive in order to average 80 (answered by ikleyn,Alan3354)

Peggy drives to town at a rate of 40 km/h. How fast must she drive BACK in order to... (answered by Alan3354,ikleyn)

liz drove off to work,leaving her house at 6;00 am. she reached her office at 8;00 am. if (answered by Cromlix)

Starting at 1 PM, Claude drove the first 27km of his bus route at 15 km/h. How fast must... (answered by josmiceli)

A girl runs at a constant speed for t minutes to run m km. How long, in second, will ita... (answered by josgarithmetic)

Please help me with this problem. Kim decides to run 2 Km in to the local gym She... (answered by ankor@dixie-net.com)