Value for which cell = x

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Hello!

I'm running into some circular logic that is making my eyes spin. I have cells A & B. Cell B contains a formula which uses A as an input. My question is: I want a cell that gives me the value B for which cell A = X (a certain value, let's say 100%).

Any thoughts?

Thanks!

M

 

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Post Cell B formula

 

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It uses visual basic:
=ErlangCsrv(D3,H3,E3,G3), where D3 is the value I am trying to find that will make this formula = 100%

CODE:

' Returns the probability a call waits.
' m is the agent count
' u is the traffic intensity
' tt is the target wait time
' tc is the average call time
Public Function ErlangCsrv(ByVal m As Integer, ByVal u As Double, ByVal tc As Double, ByVal tt As Double) As Double
    ErlangCsrv = 1 - ErlangC(m, u) * Math.Exp(-(m - u) * (tt / tc))
End Function

' Returns the probability a call waits.
' m is the agent count
' u is the traffic intensity
Public Function ErlangC(ByVal m As Integer, ByVal u As Double) As Double
    d = PowerFact(m, u)
    s = 1
    For k = 1 To m - 1
        s = s + PowerFact(k, u)
    Next k
    ' ErlangC = s
    ErlangC = d / (d + (1 - u / m) * s)
End Function

 

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Entire Code:

' By Joannes Vermorel, 2008-05-04' Copyright (c) 2008, LOKAD SAS


' Returns the probability a call waits.
' m is the agent count
' u is the traffic intensity
Public Function ErlangC(ByVal m As Integer, ByVal u As Double) As Double
    d = PowerFact(m, u)
    s = 1
    For k = 1 To m - 1
        s = s + PowerFact(k, u)
    Next k
    ' ErlangC = s
    ErlangC = d / (d + (1 - u / m) * s)
End Function


' Return Power(u, k) / Factorial(k) in a numeric-safe manner.
Public Function PowerFact(ByVal m As Integer, ByVal x As Double) As Double
    s = 0
    For k = 1 To m
        s = s + Math.Log(x / k)
    Next k
    PowerFact = Math.Exp(s)
End Function


' Returns the probability a call waits.
' m is the agent count
' u is the traffic intensity
' tt is the target wait time
' tc is the average call time
Public Function ErlangCsrv(ByVal m As Integer, ByVal u As Double, ByVal tc As Double, ByVal tt As Double) As Double
    ErlangCsrv = 1 - ErlangC(m, u) * Math.Exp(-(m - u) * (tt / tc))
End Function


' Returns the average speed of answer (ASA)
' m is the agent counts.
' u is the traffic intensity
' tc is the average call time
Public Function ASA(ByVal m As Integer, ByVal u As Double, ByVal tc As Double) As Double
    ASA = ErlangC(m, u) * tc / (m - u)
End Function

 

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Looking to solve for the value m which gives ErlangCsrv = 95%

 

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Any ideas?

 

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Thoughts?

 

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Help!

 

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bump