Anastasiya-Romanova 秀, since you are not declaring the variables (a through j), your code is running with those variables defaulting to the Variant type. While variants can be enormously useful, they should not be used here.
I ran your code unaltered and on my machine, it took 851 seconds to complete.
Since VBA is optimized for Longs, simply adding one line to your code to declare the variables (a through j) as Longs, brought the running time on my machine down to 120 seconds. So that's seven times faster just for using the appropriate variable type!
My stab at solving this puzzle in VBA runs considerably faster. In fact, it's much faster (and shorter) than anything posted thus far on this page. On my same machine, it returns all 136 correct combinations in less than one second.
There is a lot of nonsense out there (the world, the net, even here on this page!) about VBA being too slow. Don't believe it. Sure, compiled languages can be faster, but much of the time it comes down to how well you know how to handle your language. I've been programming in the BASIC language since the 1970s.
Here is my solution to the Vietnam Puzzle that I crafted for your question. Please place this in a new code module:
Option Explicit
Private z As Long, v As Variant
Public Sub Vietnam()
Dim s As String
s = "123456789"
ReDim v(1 To 200, 1 To 9)
Call FilterPermutations("", s)
[a1:i200] = v
End
End Sub
Private Sub FilterPermutations(s1 As String, s2 As String)
Dim a As Long, b As Long, c As Long, d As Long, e As Long, f As Long, _
g As Long, h As Long, i As Long, j As Long, m As Long, n As Long
n = Len(s2)
If n < 2 Then
a = Mid$(s1, 1, 1): b = Mid$(s1, 2, 1): c = Mid$(s1, 3, 1)
d = Mid$(s1, 4, 1): e = Mid$(s1, 5, 1): f = Mid$(s1, 6, 1)
g = Mid$(s1, 7, 1): h = Mid$(s1, 8, 1): i = s2
If a + (13 * b / c) + d + (12 * e) - f + (g * h / i) = 87 Then
z = z + 1
v(z, 1) = a: v(z, 2) = b: v(z, 3) = c
v(z, 4) = d: v(z, 5) = e: v(z, 6) = f
v(z, 7) = g: v(z, 8) = h: v(z, 9) = i
End If
Else
For m = 1 To n
FilterPermutations s1 + Mid$(s2, m, 1), Left$(s2, m - 1) + Right$(s2, n - m)
Next
End If
End Sub
Method #2:
Anastasiya, I will try to explain it later today, when I have more time. But in the meantime, please examine my next stab at this. It is now even shorter and completes in about 1/10th of a second. I am now using Heap's Permutation Algorithm:
Option Explicit
Private z As Long, v As Variant
Public Sub VietnamHeap()
Dim a(0 To 8) As Long
a(0) = 1: a(1) = 2: a(2) = 3: a(3) = 4: a(4) = 5: a(5) = 6: a(6) = 7: a(7) = 8: a(8) = 9
ReDim v(1 To 200, 1 To 9)
Generate 9, a
[a1:i200] = v
End
End Sub
Sub Generate(n As Long, a() As Long)
Dim t As Long, i As Long
If n = 1 Then
If a(0) + (13 * a(1) / a(2)) + a(3) + (12 * a(4)) - a(5) + (a(6) * a(7) / a(8)) = 87 Then
z = z + 1
For i = 1 To 9: v(z, i) = a(i - 1): Next
End If
Else
For i = 0 To n - 2
Generate n - 1, a
If n Mod 2 = 1 Then
t = a(0): a(0) = a(n - 1): a(n - 1) = t
Else
t = a(i): a(i) = a(n - 1): a(n - 1) = t
End If
Next
Generate n - 1, a
End If
End Sub
Method #3
And here is an even shorter version. Can anyone come up with either a shorter version or a quicker version?
Const q = 9
Dim z As Long, v(1 To 999, 1 To q)
Public Sub VietnamHeap()
Dim a(1 To q) As Long
For z = 1 To q: a(z) = z: Next: z = 0
Gen q, a
[a1].Resize(UBound(v), q) = v: End
End Sub
Sub Gen(n As Long, a() As Long)
Dim i As Long, k As Long, t As Long
If n > 1 Then
For i = 1 To n - 1
Gen n - 1, a
If n Mod 2 = 1 Then k = 1 Else k = i
t = a(k): a(k) = a(n): a(n) = t
Next
Gen n - 1, a
Else
If 87 = a(1) + 13 * a(2) / a(3) + a(4) + 12 * a(5) - a(6) + a(7) * a(8) / a(9) Then z = z + 1: For i = 1 To q: v(z, i) = a(i): Next
End If
End Sub
