There are 2 ways to roll a 3 with 2 die:
2 and 1
1 and 2

There are 5 ways to roll a 8 with 2 die:
2 and 6
6 and 2
3 and 5
5 and 3
4 and 4

The total number of possible outcomes from rolling 2 die are:
6*6 = 36

So, the probability of getting any one of the 7 favorable outcomes is: 

7/36 = 0.1944

The probability that the sum of the numbers rolled is either a 3 or an 8 is 0.1944.