
Algebra PreAlgebra and Basic Algebra Math Forum 
 LinkBack  Thread Tools  Display Modes 
July 19th, 2014, 05:28 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, 06:16 AM  #2 
Global Moderator Joined: Oct 2008 From: London, Ontario, Canada  The Forest City Posts: 7,881 Thanks: 1088 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, 06:30 AM  #3 
Global Moderator Joined: Oct 2008 From: London, Ontario, Canada  The Forest City Posts: 7,881 Thanks: 1088 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 06:45 AM. 
July 20th, 2014, 04:41 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, 12:03 PM  #5 
Global Moderator Joined: Oct 2008 From: London, Ontario, Canada  The Forest City Posts: 7,881 Thanks: 1088 Math Focus: Elementary mathematics and beyond 
40/v > 40/(v + 40) 105/v < 105/(v 10) 

Tags 
car, speed 
Thread Tools  
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Find the ratio of his usual speed to his speed that day.  Chikis  Elementary Math  25  January 31st, 2014 09:48 AM 
speed  gaussrelatz  Calculus  2  September 21st, 2013 07:48 AM 
How do I find the speed?  RealMadrid  Physics  4  September 10th, 2012 08:59 PM 
Jet Speed & Wind Speed  RMG46  Algebra  1  July 5th, 2010 10:36 PM 
Speed problem  alikn  Physics  3  October 30th, 2009 04:53 PM 