Math Is Fun Forum

Sarah Rebekah18

Member

From: Alabama

Registered: 2014-12-08

Posts: 25

Canceling fractions before multipying?

Hello I need some help? its 3 over 7 x 21 you have to cancel it before u can multiply

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I'm a gift from God <3

Bob

Administrator

Registered: 2010-06-20

Posts: 10,621

Re: Canceling fractions before multipying?

hi Sarah Rebekah18

Do you mean

In the next post, bobbym has shown what you do if that 21 is on the bottom. 

You need to make your questions clearer eg. (3 over 7) x 21     OR     3 over (7 x 21)

Be careful here, because both 3 and 7 go into 21.

But the rule is you may cancel a factor from the 'top' with one from the 'bottom'.  So you can cancel a factor of 7 but not a factor of 3.

You can always check by multiplying but it makes the arithmetic harder.:

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Canceling fractions before multipying?

Hi Sarah;

When you factor the 21 you get 3 x 7

You then cancel things that are on top with things that are same on the bottom.

Replace the canceled numbers with a 1.

Now 7 x 7 is 49.

Can I help more?

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

Sarah Rebekah18

Member

From: Alabama

Registered: 2014-12-08

Posts: 25

Re: Canceling fractions before multipying?

Okay.

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I'm a gift from God <3

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Canceling fractions before multipying?

I made some changes to the post above, please check them.

---

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

Sarah Rebekah18

Member

From: Alabama

Registered: 2014-12-08

Posts: 25

Re: Canceling fractions before multipying?

Okay!

---

I'm a gift from God <3

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Canceling fractions before multipying?

All okay?

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.