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July 19th, 2014, 04:28 AM   #1
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speed of a car!

question is attached!

i tried solving it like this:
Mack's time : 40/v
Jack's time : 40/v+40 -1/3

therefore,

40/v = 40/v+40 -1/3
i solved for v and ended up with:
v = 79.5 and 0.503

i obviously did something wrong because the answer at the back of the book is 52.11kmph
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July 19th, 2014, 05:16 AM   #2
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Let M be Mack's speed and t be time in hours.
40 = (M + 40)(t - 1/3)
40 = tM, t = 40/M

Now solve 40 = (M + 40)(40/M - 1/3) for M.
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July 19th, 2014, 05:30 AM   #3
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You have an error in your expression for Jack's time; it should simply be 40/(v + 40).
I'll assume you intended 40/v = 40/(v + 40) + 1/3, which gives a correct result.

Last edited by greg1313; July 19th, 2014 at 05:45 AM.
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July 20th, 2014, 03:41 AM   #4
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Quote:
Originally Posted by greg1313 View Post
You have an error in your expression for Jack's time; it should simply be 40/(v + 40).
I'll assume you intended 40/v = 40/(v + 40) + 1/3, which gives a correct result.
im still not sure why this question has 1/3 added onto the 40/(v+40)

40/v = 40/(v + 40) + 1/3

But from the other problem i posted (the 'trains problem') the 1/2 is subtracted from the 105/(x-10)

105/x = 105/(x-10) -1/2

why is the 1/2 subtracted? and why is the 1/3 added? arent the questions sort of the same?
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July 20th, 2014, 11:03 AM   #5
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40/v > 40/(v + 40)

105/v < 105/(v -10)
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