it'll be easiest if you make both fractions have a common denominator
the lowest common multiple of 5 and 6 is 30 so multiply 3/5 by 6 and 1/6 by 5
(3/5)*6 = 18/30 (1/6)*5 = 5/30
now add the two fractions
18/30 + 5/30 = 23/30
this is how much has been eaten. to find what fraction is left, subtract 23/30 from a full box of raisins (30/30 or 1, they mean the same thing)
this gives you 7/30