I would want to multiply 3/5 by 2 to get both equations compatible. Then add them together. 3/5 times 2 is 6/10. Then add 6 + 3 = 9. The fence is 9/10 painted. 
See the following:
3/5 + 3/10
Put 3/5 + 3/10 over the common denominator 10. 3/5 + 3/10  =  (2×3)/10 + 3/10:
(2×3)/10 + 3/10
2×3  =  6:
6/10 + 3/10
6/10 + 3/10  =  (6 + 3)/10:
(6 + 3)/10
6 + 3 = 9:
Answer: 
 9/10