Below are all 36 possible rolls.  The 5 of them that total 8
are in red.  So we should expect the ths of the
rolls should total 8.  So the answer is ths of 450,
That's 62.5 rolls.  Of course you will never expect to ever
have "half a roll". What that means is if you rolled them
490 times each day for a year, you would expect the average
number of rolls totaling 8 to be 62.5. 

(1,1) (1,2) (1,3) (1,4) (1,5) (1,6)

(2,1) (2,2) (2,3) (2,4) (2,5) (2,6)
 
(3,1) (3,2) (3,3) (3,4) (3,5) (3,6)
 
(4,1) (4,2) (4,3) (4,4) (4,5) (4,6)
 
(5,1) (5,2) (5,3) (5,4) (5,5) (5,6)
 
(6,1) (6,2) (6,3) (6,4) (6,5) (6,6)



Edwin