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Re: A driver completed the first 20 miles of a 40-mile trip at an average
[#permalink]
21 Jan 2014, 17:16
21 Jan 2014, 17:16
aeglorre
sem
A driver completed the first 20 miles of a 40-mile trip at an average speed of 50 miles per hour. At what average speed must the driver complete the remaining 20 miles to achieve an average speed of 60 miles per hour for the entire 40-mile trip? (Assume that the driver did not make any stops during the 40-mile trip.)
A. 65 mph
B. 68 mph
C. 70 mph
D. 75 mph
E. 80 mph
A. 65 mph
B. 68 mph
C. 70 mph
D. 75 mph
E. 80 mph
Basically, the first 20 miles took 24 minutes, so the second 20 miles need to take 16 minutes in order for the average to be 60miles/h..
We need to pick an option from A-E (which we call X), that in the denominator makes 120/(16*x) = 1. Note that the value needs to be divided by 10 before it is multiplied by 16.. The only value that works is D (16 * 7.5 = 120), and thus D is the answer.
Don't even ask me how I came to solve it with this convoluted mumbo jumbo but my brain worked on full gear and at the time that I did this it made perfect sense, even though Im not that good at explaining the whole process in hindsight.
why are you setting your problem to 40 miles in 40 mins? Where did 120 come from?
If you cant explain ANYTHING why write an explanation?
Re: A driver completed the first 20 miles of a 40-mile trip at an average
[#permalink]
17 Jun 2015, 08:15
17 Jun 2015, 08:15
eybrj2
A driver completed the first 20 miles of a 40 miles trip at an average speed of 50 miles per hour. At what average speed must the driver complete the remaining 20 miles to achieve an average speed of 60 miles per hour for the entire 40-miles trip? ( Assume that the driver did not make any stops during the 40-miles trip)
A. 65
B. 68
C. 70
D. 75
E. 80
A. 65
B. 68
C. 70
D. 75
E. 80
Why not 70?
50 mph + x mph / 2 = 60 mph, so x = 70 since the first 20 miles ans the other 20 miles are the same distnace.
What's wrong with my reasoning?
50 mph + x mph / 2 = 60 mph, so x = 70 since the first 20 miles ans the other 20 miles are the same distnace.
What's wrong with my reasoning?
Can we use this formula:
60 = 20/50 + 20/x ?? If yes, what is the way to solve for x ?
Re: A driver completed the first 20 miles of a 40-mile trip at an average
[#permalink]
18 Aug 2024, 20:19
18 Aug 2024, 20:19
I solved this in 40 seconds so sharing a quick solution
When distances are same and you have travelled with u rate for x distance and y rate for another x distance avg speed is 2uv/(u+v)
We can see that the distances are same here - so that shouldnt be an issue for formula
Let v be the speed of the second leg of the journey
2(50)(v)/(50+v)= 60
Solve to get v 75
When distances are same and you have travelled with u rate for x distance and y rate for another x distance avg speed is 2uv/(u+v)
We can see that the distances are same here - so that shouldnt be an issue for formula
Let v be the speed of the second leg of the journey
2(50)(v)/(50+v)= 60
Solve to get v 75
