Option Explicit

' Linear Interpolation function from the companion CD 
' of the book "Excel for Scientists" by Billo.
Function InterpolateL(lookup_value, known_xs, known_ys)

  Dim pointer As Integer
  Dim X0 As Double
  Dim Y0 As Double
  Dim X1 As Double
  Dim Y1 As Double

  'Following line added to prevent extrapolation
  If lookup_value < Application.Min(known_xs) Or lookup_value > Application.Max(known_xs) Then
    InterpolateL = CVErr(xlErrRef): Exit Function
  End If
  
  pointer = Application.Match(lookup_value, known_xs, 1)
  X0 = known_xs(pointer)
  Y0 = known_ys(pointer)
  X1 = known_xs(pointer + 1)
  Y1 = known_ys(pointer + 1)
  InterpolateL = Y0 + (lookup_value - X0) * (Y1 - Y0) / (X1 - X0)
End Function

Freezing and Boiling Points
of Heat Transfer Fluid	
Wt% 
Ethylene 
Glycol	Freezing Point, °F
0.0	32.0     
5.0	29.4     
10.0	26.2     
15.0	22.2     
20.0	17.9     
21.0	16.8     
22.0	15.9     
23.0	14.9     
24.0	13.7     
25.0	12.7     
30.0	6.7     
40.0	-8.1     
50.0	-28.9     
60.0	-54.9