To get a sum of 3, you must roll a 1 and a 2. To get a sum of 8, you must roll a 5 and 3, or two 4's.
There are two ways of rolling a 1 and a 2, two ways of rolling a 5 and a 3, and one way to roll two 4's.
There are [tex]5^2=25[/tex] possible outcomes when rolling two such solids.
So the probability of getting a sum of 3 or 8 is [tex]\dfrac{2+2+1}{25}=\dfrac15[/tex]
