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May 18th, 2012, 10:30 PM   #1
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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.
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May 18th, 2012, 11:08 PM   #2
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Re: Pls Help.. Its about SPEED DISTANCE AND TIME.

Can it be faster?
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May 19th, 2012, 01:25 AM   #3
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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.
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August 29th, 2014, 05:56 AM   #4
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(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.
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September 4th, 2014, 09:45 AM   #5
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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
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September 4th, 2014, 10:55 AM   #6
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Quote:
Originally Posted by raju View Post
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
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September 4th, 2014, 11:55 AM   #7
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Quote:
Originally Posted by Math Message Board tutor View Post
** 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
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September 4th, 2014, 11:57 AM   #8
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average speed problem

you are just copying my answer only ,how it will become 3v2
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September 4th, 2014, 07:28 PM   #9
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Raju, ab/a+b is NOT same as ab/(a+b)
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