Math Is Fun Forum

Sven

Member

Registered: 2014-05-02

Posts: 18

Compound ratios

How does the compound ratio system work i.e if it takes 20$ per post for one horse then how much will it take for 8 horses for 4.5 posts
I understand ratio of horses is 1:8
Ratio of posts i 1: 4.5 i.e 2:9
Taking compund ratio becomes 2:72
I.e 1:36
I.e it takes 720$ but how does compunding the ratio give the amount f money required. Plz help
God bless

Last edited by Sven (2014-06-06 15:14:03)

Bob

Administrator

Registered: 2010-06-20

Posts: 10,643

Re: Compound ratios

hi Sven,

I'm not quite sure what you mean by a 'post' in relation to a horse.  That rule works when the elements of the ratio are in direct proportion.  Let's consider another example.

If 1½ squirrels can eat 1½  nuts in 1½  minutes, how many nuts will six squirrels eat in six minutes ?

Assuming squirrels eat at a steady rate, having four times as many squirrels should mean they can eat four times as many nuts in the same time.

ie 6 squirrels can eat 6 nuts in 1½ minutes.

Now give them longer to eat.  The time has also gone up by a factor of four, so they can eat four times as many; 6 x 4 = 24 so

6 squirrels can eat 24 nuts in 6 minutes.

But watch out for inverse proportion.

eg.

10 people can build 10 huts in 10 days.  How long will it take 20 people to build 5 huts ?

More people means less times for the job; people and time taken are in inverse proportion.

So twice as many people means half as long to do the job:  20 people can build 10 huts in 5 days.

But there are less huts to build.  Huts and time taken are in direct proportion: so half as many huts means half as long to do the job:

20 people can build 5 huts in 2½ days.

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