Your answer 1 : 3 is correct.

[u][u][u][u]    [u][u][u][u][u]   = 9u

<—12s—>     <–6s + 3b—>

12 shirts = 4 units

1 shirt = 1/3 units

6(1/3units) = 2units => 3 blouses = 3 units => 1 blouse = 1 unit

    shirt      :    blouse

  (1/3)u     :      1u

         1        :      3

or

Alternative 1:

12 shirts = 4 units => 6 shirts = 2 units => 3 blouses = 3 units => 6 blouses = 6 units

So

shirt      :       blouse

  2u        :          6u

   1         :            3

Alternative 2:

Split the units into smaller parts, 9ux2 = 18u

[u][u][u][u][u][u][u][u]  [u][u][u][u][u][u][u][u][u][u]      = 18u

<———–12s———->  <—6s—–><——-3b———>

So if 

12s = 8u  => 6s = 4u

3b = 6u  => 6b = 12u

shirt : blouse

   4u     :    12u

    1       :     3

All roads lead to Rome…. 🙂