Six numbers 1, 2, 3, 4, 5 and 6 are written on different faces of a dice. Three different positions of the same dice are shown.
Select the number on the face opposite to the one having 1.

Let's analyze the adjacent faces from the given positions of the dice:
From position 1 (faces 1, 2, 3) and position 2 (faces 1, 3, 5):
The common faces are 1 and 3. When two faces are common in two positions of a dice, the remaining third faces are opposite to each other.
Therefore, the face 2 is opposite to 5.

From position 2 (faces 1, 3, 5) and position 3 (faces 3, 5, 6):
The common faces are 3 and 5. By the same rule, the remaining third faces are opposite to each other.
Therefore, the face 1 is opposite to 6.

Hence, the number on the face opposite to 1 is 6.