3.

[tex]6 \frac{13}{15} - 4 \frac{8}{15} < 2 \frac{7}{15} \\ 2 \frac{5}{15} < 2 \frac{7}{15} \\ \frac{5}{15} < \frac{7}{15} \\ 5 < 7[/tex]

4.

[tex]x + 3 \frac{23}{26} = 12 \frac{15}{26} \\ x = 12 \frac{15}{26} - 3 \frac{23}{26} \\ x = 11 \frac{41}{26} - 3 \frac{23}{26} \\ x = 8 \frac{18}{26} \\ x = 8 \frac{9}{13} [/tex]