My Math Forum Pls Help.. Its about SPEED DISTANCE AND TIME.

 Elementary Math Fractions, Percentages, Word Problems, Equations, Inequations, Factorization, Expansion

 May 18th, 2012, 10:30 PM #1 Newbie   Joined: May 2012 Posts: 2 Thanks: 0 Pls Help.. Its about SPEED DISTANCE AND TIME. (A) A car moves through 20 km at a speed of 40km/hr and the next 20 km at a speed of 60km/hr. Calculate its average speed. (B) A boy leaves his house at 9:30 a.m. for his school. The school is 2km away and classes start at 10.00a.m. If he walks at a speed of 3km/hr for the first kilometer, at what speed should he walk the second kilometer to reach just in time? Pls solve fast.
 May 18th, 2012, 11:08 PM #2 Newbie   Joined: May 2012 Posts: 2 Thanks: 0 Re: Pls Help.. Its about SPEED DISTANCE AND TIME. Can it be faster?
 May 19th, 2012, 01:25 AM #3 Math Team   Joined: Apr 2010 Posts: 2,780 Thanks: 361 Re: Pls Help.. Its about SPEED DISTANCE AND TIME. (A) http://en.wikipedia.org/wiki/Harmonic_mean I'll start. The first 20 km took it 1/2 hours. The second 20 km took it 1/3 hour. So the car moved 40 km in (1/2 + 1/3 = 5/6) hours. (B) The first kilometer took him 1/3 hours. So 1/6 hours to go to cover another km.
 August 29th, 2014, 05:56 AM #4 Newbie   Joined: Aug 2014 From: South Wales Posts: 5 Thanks: 0 (A) If the car covers a total of 40 km in 50 mins then the car covers 40 / 50 = 0.8 km in one minute. Multiply this by 60 to get 48 km per 60 minutes, or 48 km per hour.
 September 4th, 2014, 09:45 AM #5 Newbie   Joined: Sep 2014 From: hydedrabad Posts: 22 Thanks: 4 To calculate average speed for the above case a general formula is there: average speed=2v1v2/v1+v2 =2*40*60/100 =48 Therefore average speed of the car=48km/hr
September 4th, 2014, 10:55 AM   #6
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Quote:
 Originally Posted by raju To calculate average speed for the above case a general formula is there: average speed=2v1v2/v1+v2 =2*40*60/100 $\displaystyle \ \ \$** =48 Therefore average speed of the car=48km/hr
** No, it doesn't.

2v1v2/v1+v2 =

2v2+v2 =

3v2

--------------------------------------------------------------

You must have grouping symbols, such as parentheses:

2*v1*v2/(v1 + v2) =

2*40*60/(40 + 60) =

4800/100 =

48

September 4th, 2014, 11:55 AM   #7
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Quote:
 Originally Posted by Math Message Board tutor ** No, it doesn't. 2v1v2/v1+v2 = 2v2+v2 = 3v2 -------------------------------------------------------------- You must have grouping symbols, such as parentheses: 2*v1*v2/(v1 + v2) = 2*40*60/(40 + 60) = 4800/100 = 48
how it will become 3v2,you are just copying my answer

 September 4th, 2014, 11:57 AM #8 Newbie   Joined: Sep 2014 From: hydedrabad Posts: 22 Thanks: 4 average speed problem you are just copying my answer only ,how it will become 3v2
 September 4th, 2014, 07:28 PM #9 Math Team   Joined: Oct 2011 From: Ottawa Ontario, Canada Posts: 14,597 Thanks: 1038 Raju, ab/a+b is NOT same as ab/(a+b) Thanks from Math Message Board tutor

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