C+d-8,3

Substitute c=9 4/5 and d=-1 9/20 into the equation:

9 4/5 + (-1 9/20) - 8.3

Convert mixed fractions to improper fractions:

9 4/5 = (95 + 4)/5 = 49/5
-1 9/20 = (-120 + 9)/20 = -31/20

Substitute back into the equation:

49/5 - 31/20 - 8.3

Find a common denominator for 5 and 20, which is 20:

(49*4)/20 - 31/20 - 8.3
196/20 - 31/20 - 8.3

Combine like terms:

(196 - 31)/20 - 8.3
165/20 - 8.3

Convert the fraction to a mixed number:

165 ÷ 20 = 8 with a remainder of 5

So, the final answer is:

8 5/20 - 8.3
8 1/4 - 8.3 = -0.05