Rearrange equation to solve for a different variable - excel

Question

I am looking at VBA code (function) written by someone else.
Here is the code:
Function EuropeanDelta(StrikePrice, MarketPrice, Volatility, InterestRate As Double, PC As String, ValueDate, ExpiryDate As Date, Optional PriceOrYield As String = "P") As Double
Rem Declare our working variables
Dim r As Double
Dim d1 As Double
Dim d2 As Double
Dim t As Double
Dim SqT As Double
Rem End of variable declaration
If PriceOrYield = "Y" Then
MarketPrice = 100 - MarketPrice
StrikePrice = 100 - StrikePrice
If PC = "C" Then
PC = "P"
Else
PC = "C"
End If
End If
Rem Initiase our working variables
t = (ExpiryDate - ValueDate) / 365
SqT = Sqr(t)
r = Application.WorksheetFunction.Ln(1 + InterestRate)
d1 = (Application.WorksheetFunction.Ln(MarketPrice / StrikePrice) + (Volatility * Volatility * 0.5) * t) / (Volatility * SqT)
Rem Quick logic to deal with Calls or Puts
If PC = "C" Then
EuropeanDelta = Exp(-r * t) * Application.WorksheetFunction.NormSDist(d1)
Else
EuropeanDelta = -Exp(-r * t) * Application.WorksheetFunction.NormSDist(-d1)
End If
If PriceOrYield = "Y" Then
EuropeanDelta = EuropeanDelta * -1
End If
End Function
The whole problem is based around the line for "d1". I would like to re-organise to solve for "StrikePrice". I have tried writing it out mathematically and then re-arranging, then swapping back to VBA.

Answer 1

#duffymo is correct, but am giving the answer directly in terms of VBA code
' d1 = (Log(MarketPrice / StrikePrice) + (Volatility * Volatility * 0.5) * t) / (Volatility * Sqr(t))
'
' Volatility * Sqr(t) * d1 = Log(MarketPrice / StrikePrice) + Volatility^2 * t/2
'
' Log(MarketPrice / StrikePrice) = Volatility * Sqr(t) * d1 - Volatility^2 * t/2
'
' MarketPrice / StrikePrice = Exp(Volatility * Sqr(t) * d1 - Volatility^2 * t/2)
'
StrikePrice = MarketPrice / Exp(Volatility * Sqr(t) * d1 - Volatility^2 * t/2)
Other Notes :
For brevity replace Application.WorksheetFunction.Ln() with Log()
There is no need cache SqT = Sqr(t) since it is only used once.
For clarity replace Volatility*Volatility with Volatility^2 as internally it does the same thing.

Answer 2

This is just algebra - high school math.
Take it in steps. Make sure you do the same operation to both sides to make sure that equality still holds.
Here's your starting equation:
d = {ln(m/s) + v*v*t/2}/(v*sqrt(t))
Multiply both sides by the denominator of the RHS:
d*v*sqrt(t) = ln(m/s) + v*v*t/2
Subtract v*v*t/2 from both sides:
(d*v*sqrt(t) - v*v*t/2) = ln(m/s)
Apply the exponential function to both sides, noting that exp(ln(x)) = x:
exp(d*v*sqrt(t) - v*v*t/2) = m/s
Multiply both sides by s:
s*exp(d*v*sqrt(t) - v*v*t/2) = m
Divide both sides by exp(d*v*sqrt(t) - v*v*t/2) to get the desired result:
s = m/exp(d*v*sqrt(t) - v*v*t/2)
Let's see if this function makes sense.
At t = 0 the denominator exp(0) = 1, so the strike price is equal to the market price.
As t -> infinity, we hope that the denominator gets large so s -> zero. L'Hospital's Rule will help here.