Mathematicians please help!

[Iamcazzy]

I was never any good at percentages or ratios at school so I'd be very grateful if someone could help with the following problem.
My niece pays �8.42 per month to one company and �12.73 per month to another company. The total monthly money available is �21.15 and the payments have been worked out on a pro-rata basis.
Let's say that the company that receives �8.42 per month demands an increase to �16.00 per month.
How do I then calculate how much more she needs to pay the company that currently receives �12.73 to ensure that it to receives a pro-rata payment ie if the �8.42 company receives a further �7.58, what should the other company receive? It can't be �7.58 too can it?
I'd be very grateful if someone could show me the workings of this calculation please.

Thank you.

[jennyjen02]

Should the other company receive an extra �11.46?

Don't quite understand the situation but I think I may have answered it!?

[Iamcazzy]

Thanks jennyjen02. How did you work that out?

[Buenchico]

It's not how much that has been added on to A's payment that matters. It's what it's been multiplied by:

To get from the 'old' payment, of �8.42, to the 'new' one, of �16.00, company A has increased their demand by a factor of (16.00 � 8.42) = 1.900 (to 3 decimal places).

So company B's payment also needs to be multiplied by that factor:
�12.73 x 1.900 = �24.19 (to the nearest penny).

Chris

[jennyjen02]

I honestly don't know if that's right though, I don't know anything about finances. I don't know what pro-rata means or anything! But going by what you've told me...

The extra �7.58 is 90% of the original �8.42
So 90% of �12.73 is �11.46.

Don't know if I've done the right calculations though. x

[jennyjen02]

Just saw Chris's answer - go with him! hahah

[jennyjen02]

Wait - Chris has done the EXACT same thing, just different method, cause if you add �11.46 to �12.73, you indeed get �24.19, as was the answer given.

[Iamcazzy]

Chris, I'm immensely grateful. For a while, I thought I might not have all the information necessary to work it out but I can see it now.
Thank you very much indeed.

[Buenchico]

I think JennyJen02 must have been one of my pupils ;-)
I often found that the girls were perfectly capable of doing maths at least as well as the boys (if not much better) but they didn't believe that they actually could!

(Trust your instincts, JennyJen02! You're obviously a mathematical genius who simply refuses to believe it!)
;-)

Chris

[Iamcazzy]

jennyjen02, it looks like we've both learnt something tonight! Thanks again!

[TCL-MUMPING]

One answer has been calculated using the ratio of the the amounts paid to EACH company and multiplyingthat by the �16; the other used the ratio of the amounts paid to ONE (�16 & �8.62) and multiplied that by the �12.73.

If the original rents are A & B and A1 is the new rent,

B1 can be (A/A1) X B OR (A/B) X A1