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 Posted: Sun Sep 23, 2007 10:35 pm

Joined: Fri Sep 07, 2007 1:12 pm
Posts: 151
I was wondering if anyone has any tips/formula for working out Lowest common multiples, highest common factors etc. My daughter gets the right answer most of the time but takes a long while doing it.

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 Post subject: Posted: Mon Sep 24, 2007 8:04 am

Joined: Thu Jan 12, 2006 3:29 pm
Posts: 625
Hi Silver29

The quickest way if the number can not be identified immediately is to use prime factors

i.e 2, 3, 5, 7, 11 etc

Any number can be written as a product of their primes

i.e 6 can be made by multiplying 2 x 3 (both prime numbers)

Work out prime factors as follows:

28
divide by 2
14
divide by 2
7
is a prime number

28 is 2 x 2 x 7

36
divide by 2
18
divide by 2
9
divide by 3
3
is a prime number

36 is 2 x 2 x 3 x 3

Place both sets of numbers closer together

28 = 2 x 2 x 7
36 = 2 x 2 x 3 x 3

The highest common factor is the highest number that goes exactly into both numbers.

The common factors in the example are 2 x 2
Therefore the highest common factor is 4
4 goes into 28, 7 times
4 goes into 36, 9 times

The lowest common multiple is the lowest number that both the numbers will go into.

For lowest common multiple we only count a common value once

28 = 2 x 2 x 7
36 = 2 x 2 x 3 x 3

The lowest common multiple is

2 x 2 x 3 x 3 x 7 = 252

28 goes into 252, 9 times
36 goes into 252, 7 times

Hope this helps

Regards

Mike

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 Post subject: Posted: Mon Sep 24, 2007 11:29 am

Joined: Fri Sep 07, 2007 1:12 pm
Posts: 151
Mike, thanks so much for the step by step explanation - it really makes sense now.

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