The numbered cubes might be referring the dice. We know that there are 36 ways of dice will occur.
Take the unique list of the following pair for which the sum is less than 6, and these are
(1,1) (1,2) (1,3) (1,4) (2,1) (2,2) (2,3) (3,1) (3,2) (4,1)
Therefore there are 10 possible ways we could get.
In probability, it is a ratio of the successful outcomes over the total number of possible outcomes.
P = 10/36 = 5/18