A12-sided die is rolled. the set of equally-likely outcomes is {1,2,3,4,5,6,7,8,9,10,11,12}. find the probability of rolling a 7 or a 10.

P(A or B)=P(A)+P(B)
P(A)=1/12
P(B) =1/12
1/12 +1/12 = 2/12 =1/6

Well if there are 12 outcomes and all are  different lets think.
How many possibilities are there of rolling a 7?
Just one right? Right
What about 10 its the same thing so only 1 possibility. 
So for rolling a 7 its 1/12= unlikely same with 10
so for rolling a 10 its 1/12= unlikely\
which then you do 1/12 +1/12 = 2/12
then if u want to simplify it's 1/6
therefore the answer is 1/6
hope this helped :)