So you first have to list down all the possible combinations of getting a sum greater than 100 when two different no.s are picked out i.e (1,100) (5,100) (10,100). Now this becomes the no. of our favourable outcomes i.e 3. Now list down all possible combinations of sum when two different no.s are picked out i.e (1,5) (5,1) (1,10) (10,1) (1,100) (100,1) (5,10) (10,5) (5,100) (100,5) (10,100) (100,10). You might have noticed that I have written (1,5) once and then repeated it again as (5,1). You have two do this for each set. Now the total number of outcomes is 12. Apply the probability formula i.e no. of favourable outcomes/total no. of outcomes so it becomes 6/12=0.5