Need Help With Fractions!!!

[annie0000]

Can anyone show me how to do this fraction showing working? It's to help m explain how to do it for my son's homework - it's been well over 30 years since I did fractions :)

the question is: Bob ran 3 1/4 laps of the park in 10 1/2 minutes - how long on average did each lap take?

we got to 21/2 x 7/1 but not sure if that's right or where to go next :)

TIA

[jennyjoan]

Annie I do believe fractions are worked out different to 30 years ago.

[annie0000]

lol, yes they were and we are now both confusing each other - i know the answer is about 3 and a bit :)

[factor-fiction]

simplify teh numbers by multiplying the distance and time by 4
So he runs 13 laps in 42 minutes.
So one lap takes 42/13 minutes.

[corylus]

Ask Bob.

[factor-fiction]

jennyjoan- I'm not aware that fractions are worked out differently to 30 years ago.

[annie0000]

thanks factor - how would ou show that and apart from making it easier, is there a logical reason why you would multiply by 4

[sandyRoe]

maybe worked out differently rather that working out differently?

[annie0000]

I've talked him into telling the teacher that he doesn't understand so that he gets it gone over properly rather than covering it up by putting the right answer down

[sandyRoe]

to bring the number of laps to a whole number?

[Prudie]

The old fashioned way you want to work out 10 1/2 divided by 3 1/4
Make them both top heavy to get
21/2 divided by 13/4
Turn the right hand side upside down and change o multiply
21/2 x 4/13
Multiply top and bottom to get 84/26
Cancel where you can (only 2 goes into top and bottom here) to get 42/13
Make that back into a mixed fraction so answer 3 and 3/13 of a minute

[Ellipsis]

Here's a way of showing factor's answer ...

3 1/4 is 13/4
10 1/2 is 21/2

so you want (21/2) / (13/4)
which is (21/2) * (4/13)
which is 84/26
which is 42/13

[factor-fiction]

I multiplied by 4 just so that both numbers would be whole numbers as these are easier to work with.
But you could just make each figure a s atop heavy fraction.
So 13/4 laps in 21/2 minutes.
To divide 21/2 by 13/4 it's easier to multiply instead by 4/13

(21/2) x (4/13) = 84/26 which simplifies to 42/13 minutes = 3 and 3/13 minutes

[factor-fiction]

Sorry, started typing then phone rang

[annie0000]

fab thank you all - that makes sense to me so i'll explain that to him

[Peter Pedant]

Prudie's is how everyone did it thirty years ago.

if you are dividing by 2/3 or whatever
you turn it upside down 3/2 in this case and multiply

Form 3, 1960. I remember it well. The master didnt bother to explain why - it was just a rule.

[ for the rule two minuses make a plus, he said, if you wanna know why ask a boy in the 6th form. even then as an eight y o I thought it was a bit of an odd way to do math ]

[Obiter]

The reason that the divisor is turned upside down is that multiplication and division are inverse operations, therefore dividing by a number is the same as multiplying by its inverse. Thus 4 divided by 1/2 is the same as 4 multiplied by 2, the answer is the same in both cases: there are 8 halves in 4 and 4 x 2 = 8

[jim360]

Seems like a pretty small park... even if Bob is reasonably fit I reckon you could only just about squeeze two football pitches inside... maybe three.