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# Don't understand division and multiplication question (BIDMAS?)

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Why bother with a post grad course - waste of time? 17-10-2016
1. Howdy .

I don't get how the attached question comes to 3.

I can see how we get 3 by multiplying the top and bottom to get .

What I don't understand is how the answer says

It looks like 28 has been divided by 7 and has been simplified. But how can this be as doesn't the 15 and 20 need to be divided by the 7?

I tried writing it out like 15 x 28 ÷ 7 x 20 which helped see the ÷ by 7 but then I got confused with the x 20.

Could someone explain the steps of how to break this down to please as it looks a lot quicker than the way I did it.

Thanks
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2. when simplifying, never multiply. Just break everything up into its prime factors and then see what you can cancel

Btw I could have broken up the 4s into 2x2 but they would still cancel. I just took a shortcut
3. I personally wpuld either divde by 10 on the top and bottom, get 42\14 which you can rewrite as 14x3\14 which then cancels to three
The way they did it is basically them same except they divide by 7 iniatially. Them you get 15x4\ 20. This can be witten as 3x5x4\4x5 (because 15= 3x5 and 20=4x5) then the 4x5 cancels off and you get three.
4. (Original post by jojo55)
Howdy .

I don't get how the attached question comes to 3.

I can see how we get 3 by multiplying the top and bottom to get .

What I don't understand is how the answer says

It looks like 28 has been divided by 7 and has been simplified. But how can this be as doesn't the 15 and 20 need to be divided by the 7?

I tried writing it out like 15 x 28 ÷ 7 x 20 which helped see the ÷ by 7 but then I got confused with the x 20.

Could someone explain the steps of how to break this down to please as it looks a lot quicker than the way I did it.

Thanks
The key is that 7×20 appears in the denominator, so you have to divide by the 20, not multiply by it. Thus write it as (15x28)÷(7x20), NOT 15x28÷7x20, now cancel the 28 and 7 to give 4: (15x4)÷20, now cancel the 15 and 20 to give 3/4: (3x4)÷4, as required.

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