If you roll two fair six sided dice why is the probability that the sum is 5 or lower

Of the 36 equally probable outcomes, 10 are less than or equal to 5, so the probability is 10/36 = 5/18.

Step-by-step explanation:

Of the 36 equally probable outcomes, 10 are less than or equal to 5, so the probability is 10/36 = 5/18.

If you want you can also use algebra:

Let R1 and R2 be discrete uniform random variables representing the outcomes of each of the two dice, and let S = R1 + R2 be the sum.