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Can some one post a few questions on boat and train problems?
Thanks
Sure, here is one:
Two trains A and B start simultaneously from stations X and Y towards each
other respectively. After meeting at a point between X and Y, train A reaches
station Y in 9 hours and train B reaches station X in 4 hours from the time
they have met each other. If the speed of train A is 36 km/hr, what is the
speed of train B?
i'm not sure about the algebra in this one because I want to answer the question in under two minutes, not 35 minutes. So I just plugged in the answers, and only 54 works.
When they met they had travelled the same number of hours. After that, A went 9 hours at 36 kmph, which means it went 324 km. That's the original distance of train B, which at 54 would have taken it 6 hours.
Since B continued after the meeting point for 4 hours, still plugging in 54, it went 216 km to get to its destination. Since that's the amount of distance train A travelled first, going 36 km/hr, it would have taken it 6 hours to make that journey.
Since they both travelled 6 hours to get to the meeting point, 54 is the right answer.
My point: with MANY of the distance and motion problems, don't forget to plug in the answers. It's often a great way to elliminate the algebra.
I made two big blunders. One was in my calculation i had B travel the remaing time in 9 hrs instead of 4 and the same with A.
Tha bigger mistake was not to plug in the values and get the answer.
MORAL OF THE STORY: READ CAREFULLY and ANSWERS ARE IMPORTANT IN GMAT NOT THE APPROACH.
I hope i remember this on D-Day.
Um, almost. Please don't think that I'm a proponant of plugging in the answers all the time. I'm not. I advocate a more sophisticated approach, studying very hard, and learning when to use the answers appropriately and when to use math. The answers are not a catch-all, and they will only get you through 2-4 problems on the overall test.
Specifically with motion problems, however, I do keep an extra eye on the answers, because it's overly easy to get into the algebra with D and Dsub1 and r and rsub1 and all that stuff. The answers MOST OF THE TIME make a problem like that more approachable.
Just a follow up question. I wan't sure that that question stated that Train A and Train B, when met, had traveled the same hours in their respective speeds. Can you please explain.
I set up the question in alegabra and then plug in the number and got B too. Actually, B and C are the only logical answer for guessing.
Just a follow up question. I wan't sure that that question stated that Train A and Train B, when met, had traveled the same hours in their respective speeds. Can you please explain.
I set up the question in alegabra and then plug in the number and got B too. Actually, B and C are the only logical answer for guessing.
Pat
Hi Pat,
Nice to see you here. The question says "Two trains A and B start simultaneously from stations X and Y towards each other respectively. After meeting at a point between X and Y..." If they start at the same time and move towards each other, we can assume without stopping, then they'd be travelling for the same time.
There definately is algebra to be done here, it's just so extensive as to be not worth it compared to what plugging in the answers can do timewise.
Remember that the GMAT doesn't care about the sophistication of your work towards getting the answer, it only cares if it's right or wrong.
Just seen this problem. Nobody seems to have offered a definitive solution. Here goes....
1. let Va and Vb be the speed of A and B respectively
2. let T be the time when they meet each other
3. let Da be the distance travelled by A when it meets B.
4. Let Tb be the time taken for B to complete its journey after passing A.
From 1: Va.(T+9) = Vb.(T+4)
Va = 36 so 36.(T+9) = Vb.(T+4) --------------[1]
{total dist travelled by A = same distance as B)
From 3 and 4: Da = 36.T = Tb.Vb = 4.Vb ----------------------[2]
and T = Vb/9 -----------------------------------------[3]
Putting [2] in [1] gives 324 = Vb.T ---------------------------[4]