quick excel formula/question - semi urgent

[Crapshoot]

Okay,
what I'm trying to do is something simple that I'm having a brain fart over, and I'd appreciate the help.

A is currently 20% of B+C. (For now, we can assume C=0, B = 100, ergo A = 20). Now imagine, where C is now 30. At this point, A is now 20/130 = 15.3% of B.
I want A % of the total to remain constant (ie, 20% here) - the catch being that any corresponding increase in A must be matched by an increase in B. In this case, for A to remain at 20% of the total, A gets another 7.5 (total 27.5) and the total of 130 gets the same 7.5 (total 137.5) - 27.5/137.5 = 20%.

What I want to do is maintain A at 20% no matter what the value of C. What is the excel formula for that?

[ThunderingHERD]

Okay,
what I'm trying to do is something simple that I'm having a brain fart over, and I'd appreciate the help.

A is currently 20% of B+C. (For now, we can assume C=0, B = 100, ergo A = 20). Now imagine, where C is now 30. At this point, A is now 20/130 = 15.3% of B.
I want A % of the total to remain constant (ie, 20% here) - the catch being that any corresponding increase in A must be matched by an increase in B. In this case, for A to remain at 20% of the total, A gets another 7.5 (total 27.5) and the total of 130 gets the same 7.5 (total 137.5) - 27.5/137.5 = 20%.

What I want to do is maintain A at 20% no matter what the value of C. What is the excel formula for that?

If you mean what I think, would it be something like:

A = C * .2
B = C - A

?

Or do you want to be able to change B as well?

[ThunderingHERD]

Whoops, I misread.

Won't A = (B + C) * .2 work, or is there something I'm missing?

[Crapshoot]

the problem is that any increase in A is matched by a corresponding increase in B - ergo, to maintain a 20% percentage, there's an increase in the numerator and the denominator. That's what is causing me irritation./

[ThunderingHERD]

the problem is that any increase in A is matched by a corresponding increase in B - ergo, to maintain a 20% percentage, there's an increase in the numerator and the denominator. That's what is causing me irritation./

OH. I get you now. :)

Let me think for a sec.

[Suburban Rhythm]

Whoops, I misread.

Won't A = (B + C) * .2 work, or is there something I'm missing?

Maybe I am misreading too...but why not make cell A a formula = (b+c)*.20 ?

No matter what you change B (or C) to, A will still equal 20% of the total.

In your example, B= 100 C = 0, A will return 20
Change C= 30, A will return 26, still 20% of the total.

I guess that I am assuming B & C will be the only items modified, and C the result of those, or do want to be able to modify A as well?

[SackAttack]

Okay,
what I'm trying to do is something simple that I'm having a brain fart over, and I'd appreciate the help.

A is currently 20% of B+C. (For now, we can assume C=0, B = 100, ergo A = 20). Now imagine, where C is now 30. At this point, A is now 20/130 = 15.3% of B.
I want A % of the total to remain constant (ie, 20% here) - the catch being that any corresponding increase in A must be matched by an increase in B. In this case, for A to remain at 20% of the total, A gets another 7.5 (total 27.5) and the total of 130 gets the same 7.5 (total 137.5) - 27.5/137.5 = 20%.

What I want to do is maintain A at 20% no matter what the value of C. What is the excel formula for that?

I'm seeing two conflicting criteria here, and possibly bad math.

A is currently 20% of B+C. Okay. Do you want it to maintain that ratio relative to B+C, or do you want it to always remain 20% of B?

If the former, If C = 0 and B = 100, as in your first example, A = 20, as you said. But if you increase C to 30, doesn't that mean that A = 26, not 20?

And in that case, doesn't A always remain at a 20% ratio to B and B+C, simultaneously?

[ThunderingHERD]

I think he's saying that whenever C changes, he needs to change the values of A and B by an equal amount while A remains 20% of (B+C).

I have no idea how to do that in Excel without using VBA.

[terpkristin]

I'm not sure I understand the problem, either.
But I'd use Solver.

/tk