Wednesday, July 12, 2017

algebra precalculus - To find the total distance between two points A and B with just speed of train and difference in distance between two trains?



Two trains approach each other at 25 km/hr and 30 km/hr respectively from two points A and B. Second Train travels 20 km more than first. What is the distance between A and B ?




My approach:



Since Distance=SpeedTime
[I just added the time it took to cover the distance taken by the Second train(30 km/hr) to cover 20 km to the First train(25 km/hr) to get distance as constant]



(First Train)



For 20 km at 25 km/hr, the time taken would be




time =2025=45




So First train taken would take 4/5 time more to cover the distance d




d=25(t+4/5) ---> (1)



d=30t ---> (2)




Now since distance is constant and speed is inversely proportional to time,



Ratio of speeds =2530=56



Ratio of times =t+4/5t=5t+45t=5t+45t



So



56=5t+45t




Since it is inversely proportional



5(5t+4)=6(5t)



25t+20=30t



25t30t=20



5t=20




t=4



So applying t=4 in (2)
d=304=120 km but its wrong



The correct answer is 220 km



I don't understand! help


Answer



I think best way to describe all the data is in a table like this




VTDTrain 125[kmh]Train 230[kmh]



Denote the distance the first train traveled until the meeting
by x. Therefore the second train traveled x+20. Lets add this
to the table:




VTDTrain 125[kmh]x[km]Train 230[kmh]x+20[km]



Now we can complete our table using that T=DV



VTDTrain 125[kmh]25x[h]x[km]Train 230[kmh]30x+20[h]x+20[km]



Assuming both trains left both points at the same time we get that
T1=T2 where T1,T2 is the time travel for each train
until the meeting. So
25x=30x+20x=100[km]
Therefore the distance between A and B is D1+D2 where D1,D2
is the distance each train traveled untill the meeting. So the distance
is
(100)+(100+20)=220[km]


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