How would you identify what to take from a word problem to put into a quadratic equation?

2 Answers | Add Yours

beckden's profile pic

beckden | High School Teacher | (Level 1) Educator

Posted on

1st of all I have lived in Grayslake.  It is all one word, and it is about 50 miles from Chicago.  Enough about Geography lets do math.

Sometimes it is easier if you create a table to write the things you know about this problem, but this format is difficult to do on this site.  Another thing you can do is to draw a picture, which again doesnt' work well on the web, but I will write it out.

The first thing I notice is I don't know the speeds of the trains but they express the speed of the express train in terms of the local.  I also don't know how long it took for them to get to Chicago, but again the express train arrives an hour before the local train.  If I could find out how long it took for each train to arrive in Chicago, I could divide 50 miles by that time and get an answer.  But first I need some variables, and since I don't like using fractions, I will pick the local train since expressing the express trains speed is twice the local trains speed, and I don't end up with one half.

Let v = speed of local train so the speed of the express train is 2v.

If t is the time it takes for the local train to get to Chicago, then t-1 is the time it takes the express train to travel to Chicago.  Since they both travel the same distance (50 miles) I get:

v*t = 2v*(t-1)   Since v cancels out I get t = 2(t-1)

Solving for t I get t = 2t - 2, -t = -2, and finally t = 2 hours.

Since I know the distance traveled this gives v = 50miles/2hours = 25 mi/hr  for the local train and twice 25 =  50 mi/hr for the express train.

My answer is 25 mi/hr for the local train, and 50 mi/hr for the express train.

intelme's profile pic

intelme | Student, Grade 9 | (Level 2) eNoter

Posted on

 

How do I put this word problem into a quadratic equation or set it up? 
An express and local train leave Gray’s Lake at 3 P.M. and head for Chicago 50 miles away. The express travels twice as fast as the local, and arrives 1 hour ahead of it. Find the speed of each train. 
I know that distance equals rate times time but on all else I am confused to say the least. Any info you can offer will be greatly appreciated.
(Scroll Down for Answer!)

 

an express and local train leave Gray's Lake at 3 P.M. and head for Chicago 50 miles away. The express travels twice as fast as the local, and arrives 1 hour ahead of it. Find the speed of each train.:The time they left is irrelevant to the problem:Let s = speed of the localthen2s = speed of the express:Write a time equation: Time = :Express time + 1 hr = Local time + 1 = 
Get rid of the denominators, mult equation by 2s; results:50 + 2s = 2(50)2s = 100 - 502s = 50s = s = 25 mph speed of the localthen50 mph speed of the express;Check solution
+ 1 = 1 + 1 = 2

 

We’ve answered 318,913 questions. We can answer yours, too.

Ask a question