Is this story problem actually possible to solve?

shabau

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Apr 4, 2018
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First, it has been a while since I've had to do any formalized math problems, and my skills are a bit rusty. I was recently presented with a story problem, and my initial impression is that it is impossible to solve with the given information (perhaps some part was left out accidentally when I was asked?). I am no expert, however, so I'd like to see if anyone more knowledgeable than me can either confirm my suspicion, or explain a bit how to get an answer. The question is as follows:

"You have two cars starting from two different points and traveling in straight lines such that their paths will cross at some point. They each travel an equal total distance from start to finish, and after their paths intersect, it takes one car 1 hour and the other car 9 hours to reach their respective destinations. How much faster is the one car compared to the other?"

My thought process is this, but please correct me if I am wrong about anything:

-> Distance = rate * time

-> We know each car travels the same distance overall, and we know a part of the total time taken. So if Ra and Rb are the cars' respective rates, then given the distance formula and what we know:

Ra(x + 1) = Rb(y + 9) [where x and y are the unknown portions of time before the intersection point of the cars]

-> Given the problem, I suppose we also know that the cars have different rates, such that:

Ra = zRb [where z essentially represents the answer to the question]

-> Now we could obviously substitute one of the rate variables to get something like:

zRb(x + 1) = Rb(y + 9)

-> But then we still essentially have 4 different unknowns to deal with. And even though we don't strictly speaking need values for Ra or Rb (as we are only technically interested in the multiple z for the answer) and can simply:

z = (y + 9)/(x + 1)

-> That still leaves x, y, and z unknown, and I just don't see a way to obtain a simple numerical value for z here given what we have.

At any rate (pun sort of intended), any thoughts or help anyone has is greatly appreciated so I can stop chasing this one around in my head. Thanks in advance!
 
Hmm good question. That wasn't explicitly stated to me when I wrote the problem down, but perhaps that was intended to be communicated or assumed as part of the problem. So if I follow you there, then we can eliminate one variable:

-> If car 1 and car 2 meet and cross paths at the same time, then the time it takes for both of them to reach the intersection point must be the same. Thus, working the problem again:

Ra(x + 1) = Rb(x + 9) [where x is the amount of time it takes for the cars to meet before they finish their respective paths]

Ra = zRb

zRb(x + 1) = Rb(x + 9)

z = (x + 9)/(x + 1) [or basically, the total travel time for one car divided by the total travel time for the other, which makes perfect sense]

But since we don't actually have the total travel time for either vehicle, we still can't get x if I'm not mistaken? It seems like we need another equation involving only z and x in order to do a substitution here, but I'm still not seeing it based on the given info... Am I missing something else?
 
Fast driver A drives at speed a, slowpoke B at speed b.
They meet after u hours.

A(@a).............au...............>(u hr.)-------a------->(1 hr.)

(9 hr)<---------9b--------------(u hr.)<.......bu........B(@b)

au = 9b : u = 9b / a [1]

a = bu : u = a / b [2]

[1][2]:
9b / a = a / b
a^2 = 9b^2
a = 3b

I've assumed they leave at SAME time...else not solvable...

Ah thank you! I was missing the fact that the initial distance covered by A would have to be the same as the 9hr distance covered by B given that they maintain a constant rate of travel. Problem solved!
 
Fast driver A drives at speed a, slowpoke B at speed b.
They meet after u hours.

A(@a).............au...............>(u hr.)-------a------->(1 hr.)

(9 hr)<---------9b--------------(u hr.)<.......bu........B(@b)

au = 9b : u = 9b / a [1]

a = bu : u = a / b [2]

[1][2]:
9b / a = a / b
a^2 = 9b^2
a = 3b

I've assumed they leave at SAME time...else not solvable...

As I read the problem, it doesn't say they are traveling on the same road, or that each one's destination is the other's start. You are making a lot of assumptions, not just one.

My impression is that their paths (not just their cars) intersect in only one place, so they are two intersecting segments, most of whose dimensions we know nothing about. All we know is that they are the same length.

If my impression is wrong, the problem is at least very unclear.
 
First, it has been a while since I've had to do any formalized math problems, and my skills are a bit rusty. I was recently presented with a story problem, and my initial impression is that it is impossible to solve with the given information (perhaps some part was left out accidentally when I was asked?). I am no expert, however, so I'd like to see if anyone more knowledgeable than me can either confirm my suspicion, or explain a bit how to get an answer. The question is as follows:

"You have two cars starting from two different points and traveling in straight lines such that their paths will cross at some point. They each travel an equal total distance from start to finish, and after their paths intersect, it takes one car 1 hour and the other car 9 hours to reach their respective destinations. How much faster is the one car compared to the other?"

My thought process is this, but please correct me if I am wrong about anything:

-> Distance = rate * time

-> We know each car travels the same distance overall, and we know a part of the total time taken. So if Ra and Rb are the cars' respective rates, then given the distance formula and what we know:

Ra(x + 1) = Rb(y + 9) [where x and y are the unknown portions of time before the intersection point of the cars]

-> Given the problem, I suppose we also know that the cars have different rates, such that:

Ra = zRb [where z essentially represents the answer to the question]

-> Now we could obviously substitute one of the rate variables to get something like:

zRb(x + 1) = Rb(y + 9)

-> But then we still essentially have 4 different unknowns to deal with. And even though we don't strictly speaking need values for Ra or Rb (as we are only technically interested in the multiple z for the answer) and can simply:

z = (y + 9)/(x + 1)

-> That still leaves x, y, and z unknown, and I just don't see a way to obtain a simple numerical value for z here given what we have.

At any rate (pun sort of intended), any thoughts or help anyone has is greatly appreciated so I can stop chasing this one around in my head. Thanks in advance!

I disagree with you saying that you just need to find z. Consider the following. Ra = 20mph and Rb=10mph. Then Ra=2Rb and z=2. However, Ra is traveling 10mph faster than Rb. Then the answer to your questions (using my numbers) would be 10mph NOT 2. Also, the z is just a number, that is by YOUR definition of z has no units. Clearly the units to the answer will be mph (or some version of that).
 
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Hmm good question. That wasn't explicitly stated to me when I wrote the problem down, but perhaps that was intended to be communicated or assumed as part of the problem. So if I follow you there, then we can eliminate one variable:

-> If car 1 and car 2 meet and cross paths at the same time, then the time it takes for both of them to reach the intersection point must be the same. Thus, working the problem again:

Ra(x + 1) = Rb(x + 9) [where x is the amount of time it takes for the cars to meet before they finish their respective paths]

Ra = zRb

zRb(x + 1) = Rb(x + 9)

z = (x + 9)/(x + 1) [or basically, the total travel time for one car divided by the total travel time for the other, which makes perfect sense]

But since we don't actually have the total travel time for either vehicle, we still can't get x if I'm not mistaken? It seems like we need another equation involving only z and x in order to do a substitution here, but I'm still not seeing it based on the given info... Am I missing something else?
The way you used x works only if the two cars started at the same time. This was not given. But since so much seems to be left out I agree that you should assume that. Just wanted you to know that it's an assumption and was not given.
 
Fast driver A drives at speed a, slowpoke B at speed b.
They meet after u hours.

A(@a).............au...............>(u hr.)-------a------->(1 hr.)

(9 hr)<---------9b--------------(u hr.)<.......bu........B(@b)

au = 9b : u = 9b / a [1]

a = bu : u = a / b [2]

[1][2]:
9b / a = a / b
a^2 = 9b^2
a = 3b

I've assumed they leave at SAME time...else not solvable...
I am not exactly sure what you did (I need to try to figure that out) but you clearly ended up with a=3b. However the problem is NOT asking how many times faster one car is going then the other car. The problem asked how much faster is one car than the other car.
 
Fast driver A drives at speed a, slowpoke B at speed b.
They meet after u hours.

A(@a).............au...............>(u hr.)-------a------->(1 hr.)

(9 hr)<---------9b--------------(u hr.)<.......bu........B(@b)

au = 9b : u = 9b / a [1]

a = bu : u = a / b [2]

[1][2]:
9b / a = a / b
a^2 = 9b^2
a = 3b

I've assumed they leave at SAME time...else not solvable...
Sir Denis: Wow, I finally see what you are saying! I almost think that we read different problems as you are assuming a great deal. But again you did not say how much faster one car is going than the other car.
 
Well, I'm 99% sure that what I'm "assuming" is what
that problem SHOULD BE if properly worded!!

Agreed, and again thanks to everyone who chimed in here. As Denis says, I think the biggest issue with this problem is the way it was phrased and presented to me. I think that "how much faster" really was intended to be a simple multiple (i.e. 3 times faster) given the context of me being asked in the first place, and that the other necessary assumptions simply weren't clearly communicated to me. From the beginning, this problem struck me as what should be a relatively simple algebra problem that got confused in translation or something. In any case, the whole situation seems a lot clearer to me now, and for that I am very grateful :D
 
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