how do I get the average of lookup value from ssrs?

=Lookup(dsc.Value,Dsc.value,Fields!Fmotion.Value,"Stage")

I want to get average of that lookup value. How can i write an expression that shows average of that lookup value?

Lookup(x,x,x,x) only returns a single value; the first match in the dataset that it finds and the average of a single value is obviously that value. If you want the average of all values that match your Lookup condition, use the LookupSet function to return multiple values. Then you can use the code behind ability to embed VB code and return an average for the LookupSet:

Function AvgLookup(ByVal items As Object()) As Decimal
If items Is Nothing Then
    Return Nothing
End If


Dim suma As Decimal = 0
Dim avga As Decimal = 0
Dim counta As Integer = 0

For Each item As Object In items
    If Not item Is Nothing Then
        counta += 1
        suma += Convert.ToDecimal(item)
    End If
Next

If counta > 0 Then
    avga = suma / counta
Else
    avga = 0
End If
Return avga
End Function

which you call in an expression on your report like:

=Code.AvgLookup( Lookupset(dsc.Value,Dsc.value,Fields!Fmotion.Value,"Stage") )