To add these fractions, we first need to find a common denominator.

For 3/13 + 5.7/13, we can rewrite 5.7 as 5.7 = 5 + 0.7 = 5 + 7/10 = 5 + 7/10 = 5 7/10 = 57/10.

So we have:
3/13 + 57/10 = 30/130 + 57/130 = 87/130.

For 6.2/17 + 3.11/17, we can rewrite 6.2 as 6.2 = 6 + 0.2 = 6 + 2/10 = 6 + 2/10 = 6 2/10 = 62/10.

So we have:
62/17 + 311/17 = 62/170 + 311/170 = 373/170.

Therefore, the final answers are:
3/13 + 5.7/13 = 87/130
6.2/17 + 3.11/17 = 373/170