The problem is that A and B as string (text) variables, and the
"+" sign is the old way of concatenating (combining) strings.
Instead, try,

Dim A As Double
Dim B As Double
Dim C As Double
A = Application.InputBox(prompt:="Enter A:", Type:=1)
B = Application.InputBox(prompt:="Enter B:", Type:=1)
C = A + B

The Type:=1 argument restricts users to entering numbers. It
prevents non-numeric entry.

Because input box returns string, not number, so conatenation of 3 and 4
gives 34.
Declare variables

dim a as double, b as double

or dim a as integer, b as integer.

It will convert it to numbers, however it will crash when user enters
not numbers.

So you can:

dim A as string, b as string

dim iA as integer, iB as integer

and using isNumber you can check if data enterd by user is a number and
after that convert it to integer

iA = cInt (A)

Try this:

A = Val(InputBox("Enter domestic investment amount"))
B = Val(InputBox("Enter foreign investment amount"))
TotalInvestment = A + B

OR
TotalInvestment = Val(A) + Val(B)

OR
A = InputBox("Enter domestic investment amount",,,,,,1)
B = InputBox("Enter foreign investment amount",,,,,,1)

Either one of these should work.  The issue is that the InputBox returns
text values, not numeric unless you specify using the last arguement of "1"
which tells VBA the data being returned is numeric.  Also, in VBA, + and &
are concatenation functions for text, which is why you were getting 34
returned.

If you use either of the first two possible solutions, you will need to
check each response to see if they are numeric or not (you could use the
IsNumeric function to do so) before using the Val() function to ensure that
the inputs are indeed numeric in nature.

Hope this helps.