SOLUTION: Having travelled halfway to the end of a journey at an average speed of 15km/hr, how fast must you traverse the rest of the trip on order to average 20 km/hr? How do I do this?

Question 793164: Having travelled halfway to the end of a journey at an average speed of 15km/hr, how fast must you traverse the rest of the trip on order to average 20 km/hr?
How do I do this? I know that simply averaging the two speeds would be incorrect.
Answer by mananth(16946) (Show Source): You can put this solution on YOUR website!
Let y be the rate in second half and x the total distance
t=d/r
time first half + time second half = time for total journey
(x/2)/(15)+ (x/2) /y = x/20
x/30 -x/20 = - x/2y
(2x-3x)/60 = -x/2y
-x/60 = -x/2y
2y=60
y=30 km/h

RELATED QUESTIONS
Please help me solve this question. A motorist travelled 20km of an average speed of... (answered by josgarithmetic)

Ivan drives to the metro station each morning and just to catch his train. He drives at... (answered by ankor@dixie-net.com)

Ivan drives to the metro station each morning and just to catch his train. He drives at... (answered by Boreal,josgarithmetic)

if you drive with a speed of 20km/hr, how fast you must go again returning by the same... (answered by ankor@dixie-net.com)

journey of 100 km in 3.5 hrs Part travelled 32 km/hr and rest at 24 km/hr how far... (answered by ewatrrr)

Hi mr lee plans to dine at a restaurant 900km away.he took a train which travelled part... (answered by josgarithmetic)

A car journey of 300km lasts 4 hours. Parts of this journey is on a freeway at an average (answered by mananth)

Danny drove halfway from town P to town Q at an average speed of 60km/h. He wants to... (answered by Alan3354)

Dear sir, Please help me with the following questions. I have solved one of them for... (answered by Alan3354)