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October 4th, 2018, 03:01 AM  #1 
Senior Member Joined: Aug 2014 From: India Posts: 332 Thanks: 1  Why first method answer didn't match second method?
On a circular track of length 100m. A and B start running clockwise from the same point with speeds of 5m/sec and 7m/sec respectively. After how much time will they meet for the first time while running? First method: Track = 100m → SA/SB = 5/7 A does 5 rounds B does 7 rounds Difference = 2; It's their second meeting. B overtakes A somewhere in the middle 5/7 Written as 2.5/3.5 A does 2.5 rounds, B does 3.5 rounds. Then difference is 1. It means faster guy “B” overtakes “A” for first time. If 2 meetings happened → 100 sec → (5*20) A → 20 sec When would they first meeting would happened. → 100/2 = 50 sec Second method: (Total distance)/(Relative distance) = 100/(7  4) = 100/3 sec After 100/3 sec, A and B meet for first time. Last edited by skipjack; October 4th, 2018 at 10:41 AM. 
October 4th, 2018, 04:03 AM  #2 
Senior Member Joined: Dec 2015 From: Earth Posts: 248 Thanks: 27 
$\displaystyle T=lcm(v_A , v_B ) = lcm(7,5)=35$ I write a formula below : If A does a round for time $\displaystyle t_1$ If B does a round for time $\displaystyle t_2$ then they meet again at the same point after time $\displaystyle lcm(t_1,t_2)$ Last edited by idontknow; October 4th, 2018 at 04:06 AM. 
October 4th, 2018, 10:01 AM  #3 
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,302 Thanks: 935 
The time t it takes for 1st meeting to occur is d / (b  a), where d = track length (100), b = faster speed (7), a = slower speed (5) at + d = bt bt  at = d t(b  a) = d t = d / (b  a) So: t = 100 / (7  5) = 50 WHY are you trying your obscure 2nd calculation? 
October 4th, 2018, 10:30 AM  #4 
Senior Member Joined: Aug 2014 From: India Posts: 332 Thanks: 1  Can I use this formula for all circular track problems?
Last edited by skipjack; October 4th, 2018 at 10:39 AM. 
October 4th, 2018, 10:43 AM  #5 
Global Moderator Joined: Dec 2006 Posts: 19,713 Thanks: 1806  
October 4th, 2018, 11:52 AM  #6 
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,302 Thanks: 935  
October 4th, 2018, 07:59 PM  #7 
Senior Member Joined: Aug 2014 From: India Posts: 332 Thanks: 1  
October 4th, 2018, 11:26 PM  #8 
Global Moderator Joined: Dec 2006 Posts: 19,713 Thanks: 1806 
They're just the steps that can be used to solve for t, given the first equation and that b doesn't equal a.

October 5th, 2018, 05:36 AM  #9 
Math Team Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 13,302 Thanks: 935  

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