Imports System.IO
Module Module1
 Sub Main()
  Dim auc As Double = 0
  Dim ba As Double = 0.45
  Dim cl As Double = 4
  Dim dose As Double = 0
  Dim f(3) As Double
  Dim g As Double = 0
  Dim h As Double = 0.01
  Dim i As Integer
  Dim ka As Double = 0.07
  Dim kn1(3), kn2(3), kn3(3), kn4(3) As Double
  Dim k12 As Double = 0.028
  Dim k21 As Double = 0.0088
  Dim pc As Double
  Dim q As Double = 10
  Dim rr As Double = 1.7 / 60
  Dim tdose As Double = 0
  Dim vd As Double = 600
  Dim w As Double = 75.3
  Dim X As Double = 0
  Dim xmax As Double = 1440
  Dim Y(3) As Double
  Dim yn(3) As Double
  For i = 1 To 3
   Y(i) = 0
  Next
  Dim file As StreamWriter
  file = My.Computer.FileSystem.OpenTextFileWriter(
            "C:\Users\Kevin C. Johnson\crnd.txt", False)
Start:
  If X >= h Then dose = rr * h
  tdose = tdose + dose
  pc = Y(2) * 1000000 / (vd * w)
  auc = auc + pc * h
  If g < h Then file.Write(X.ToString("0.00E+0"))
  If g < h Then file.Write(" ")
  If g < h Then file.WriteLine(pc.ToString("0.00E+0"))
  g = g + h
  If g > q - h / 2 Then g = 0
  For i = 1 To 3
   eq(f, rr, ka, Y, ba, cl, vd, k12, k21)
   yn(i) = Y(i)
   kn1(i) = f(i)
   X = X + h / 2
   Y(i) = yn(i) + kn1(i) * h / 2
   eq(f, rr, ka, Y, ba, cl, vd, k12, k21)
   kn2(i) = f(i)
   Y(i) = yn(i) + kn2(i) * h / 2
   eq(f, rr, ka, Y, ba, cl, vd, k12, k21)
   kn3(i) = f(i)
   X = X + h / 2
   Y(i) = yn(i) + kn3(i) * h
   eq(f, rr, ka, Y, ba, cl, vd, k12, k21)
   kn4(i) = f(i)
   Y(i) = yn(i) + h * (kn1(i) + 2 * kn2(i) + 2 * kn3(i) + kn4(i)) / 6
   X = X - h
  Next
  X = X + h
  If X < xmax + h Then GoTo Start
  file.Close()
 End Sub
 Sub eq(ByRef f() As Double, ByRef rr As Double, ByRef ka As Double,
        ByRef Y() As Double, ByRef ba As Double, ByRef cl As Double,
        ByRef vd As Double, ByRef k12 As Double, ByRef k21 As Double)
  f(1) = rr - ka * Y(1)
  f(2) = ka * Y(1) * ba - (cl / vd + k12) * Y(2) + k21 * Y(3)
  f(3) = k12 * Y(2) - k21 * Y(3)
 End Sub
End Module