Bonjour,
[tex] \frac{2}{15} + \frac{7}{5} = \frac{2}{15} + \frac{21}{15} = \frac{23}{15} [/tex]
[tex] \frac{5}{7} - \frac{31}{21} = \frac{15}{21} - \frac{31}{21} = \frac{ - 16}{21} [/tex]
[tex] \frac{3}{7} + \frac{2}{5} = \frac{15}{35} + \frac{14}{35} = \frac{29}{35} [/tex]