Answer :

18 5/12 + 8 10/17 + 23 7/12 + 28 7/17

First change them to improper fractions

18 5/12 = 221/12

8 10/17 = 146/17

23 7/12 = 283/12

28 7/17 = 483/17

now find the common denominator which would be  204

221/12 =  3757 /204

146/17 =    1752 /204

283/ 12 =    4811 /204

483/ 17 =      5796 /204

now add

(3757 + 1752 + 4811 + 5796) = 16,116/204

now divide

16,116 ÷  204 =  79

Your answer is D. 79

Hope this helps. :)