Two positions of the same cubical block are shown. When '6' is at the top, which number will be at the bottom?

4

Step 1) In figure Ist and IInd, 4 is common

Step 2) The number ‘4’ is common in both the dice, but it is at a different position. In order to find out, the number appearing on the opposite face, rotate the dice in a clockwise direction with the common number as the reference.

Step 3)   

4   5   2

4   3   1

Step 4)   5  is opposite to 3

               2  is  opposite  to 1

Therefore ,

              6 will be opposite to 4