What is the probability that a roll of a pair of 6-sided dice will sum to 8?

I'm assuming that the 6-sided dice have the numbers "1" through "6" on the faces. If that assumption holds, then there are 5 ways to add to 8. Those ways are listed below:
2+6 = 8
3+5 = 8
4+4 = 8
5+3 = 8
6+2 = 8

This is out of 6*6 = 36 total ways to roll the two dice. It might help to draw out a dice chart to list all the possible ways. 

There are 5 ways to get what we want (sum to 8) out of 36 ways total. Divide the two values to get 5/36 which is the answer as a fraction

To convert to decimal form, use long division or a calculator to get 5/36 = 0.1389 which is approximate (rounded to four decimal places)

If you want a percentage, then move the decimal 2 spots to the right to go from 0.1389 to 13.89%