Ben rolls two number cubes. what is the probability that the sum of the numbers he rolls is less than 6

There are 6 faces on each dice and the total possible combinations possible = 6*6 = 36 combinations.

Of those 36 combinations, the combinations of sum less than 6  are:
1&1  2&1 3&1 4&1
1&2  2&2 3&2
1&3  2&3
1&4

Thus, 10/36 :-)