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July 19th, 2014, 04:28 AM  #1 
Newbie Joined: Jul 2014 From: Australia Posts: 24 Thanks: 0  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 
July 19th, 2014, 05:16 AM  #2 
Global Moderator Joined: Oct 2008 From: London, Ontario, Canada  The Forest City Posts: 7,859 Thanks: 1080 Math Focus: Elementary mathematics and beyond 
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. 
July 19th, 2014, 05:30 AM  #3 
Global Moderator Joined: Oct 2008 From: London, Ontario, Canada  The Forest City Posts: 7,859 Thanks: 1080 Math Focus: Elementary mathematics and beyond 
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. 
July 20th, 2014, 03:41 AM  #4  
Newbie Joined: Jul 2014 From: Australia Posts: 24 Thanks: 0  Quote:
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/(x10) 105/x = 105/(x10) 1/2 why is the 1/2 subtracted? and why is the 1/3 added? arent the questions sort of the same?  
July 20th, 2014, 11:03 AM  #5 
Global Moderator Joined: Oct 2008 From: London, Ontario, Canada  The Forest City Posts: 7,859 Thanks: 1080 Math Focus: Elementary mathematics and beyond 
40/v > 40/(v + 40) 105/v < 105/(v 10) 

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