
Elementary Math Fractions, Percentages, Word Problems, Equations, Inequations, Factorization, Expansion 
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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  
Banned Camp Joined: Jun 2014 From: Earth Posts: 945 Thanks: 191  Quote:
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 
Newbie Joined: Sep 2014 From: hydedrabad Posts: 22 Thanks: 4  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)


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