10.2 + (-42 1/3) + (-3 13/15)

First, convert all the numbers to fractions:
10.2 = 10 1/5
-42 1/3 = -127/3
-3 13/15 = -58/15

Now, add the fractions together:
10 1/5 - 127/3 - 58/15

To add the fractions, we need to find a common denominator. The least common multiple of 3 and 15 is 15. So, we can convert 10 1/5 into an improper fraction:
10 1/5 = 51/5

Now put all the fractions over 15:
51/5 = 153/15

Now we can add the fractions:
153/15 - 127/3 - 58/15 = -132/15

Now convert the result back to a mixed number:
-132/15 = -8 12/15 = -8 4/5

Therefore, 10.2 + (-42 1/3) + (-3 13/15) = -8 4/5.