Given that the distance between any two points in A, B and C is 1km, ABC forms an equilateral triangle (as shown in the figure).

Because △ABC is an equilateral triangle

Therefore ∠ A = ∠ B = ∠ C = 60.

Known ∠BDC=α

So ∠ Abd = α-60.

△ABD has sine theorem: AD/sin∠ABD=AB/sin∠ADB.

That is, ad/sin (α-60) =1/sin (180-α).

Then ad = sin (α-60)/sinα.

Similarly, there is the sine theorem in △BCD: BC/SIN ∠ BDC = BD/SIN ∠ C.

That is, 1/sinα = BD/sin60.

Therefore, BD = sin60/sinα.

Then the travel distance of all employees is S= 100*AD+400*BD.

=[ 100 sin(α-60)/sinα]+[400 * sin 60/sinα]

Because point d is different from a and c on AC, so:

α∈(60 , 120 )

It is 60 when it is infinitely close to point A and 120 when it is infinitely close to point C.