Решение:

a)

$$\frac{2}{9} + \frac{3}{7} = \frac{2*7 + 3*9}{9*7} = \frac{14 + 27}{63} = \frac{41}{63}$$

б)

$$\frac{7}{8} + \frac{3}{4} = \frac{7}{8} + \frac{3*2}{4*2} = \frac{7}{8} + \frac{6}{8} = \frac{7+6}{8} = \frac{13}{8} = 1\frac{5}{8}$$

в)

$$\frac{5}{24} + \frac{25}{36} = \frac{5*3 + 25*2}{72} = \frac{15 + 50}{72} = \frac{65}{72}$$

г)

$$\frac{59}{60} + \frac{39}{40} = \frac{59*2 + 39*3}{120} = \frac{118 + 117}{120} = \frac{235}{120} = \frac{47}{24} = 1\frac{23}{24}$$

д)

$$3\frac{1}{8} + 2\frac{1}{7} = 3 + \frac{1}{8} + 2 + \frac{1}{7} = 5 + \frac{1}{8} + \frac{1}{7} = 5 + \frac{1*7 + 1*8}{56} = 5 + \frac{7 + 8}{56} = 5 + \frac{15}{56} = 5\frac{15}{56}$$

e)

$$5\frac{1}{6} + 1\frac{2}{3} = 5 + \frac{1}{6} + 1 + \frac{2}{3} = 6 + \frac{1}{6} + \frac{2}{3} = 6 + \frac{1}{6} + \frac{2*2}{3*2} = 6 + \frac{1}{6} + \frac{4}{6} = 6 + \frac{1+4}{6} = 6 + \frac{5}{6} = 6\frac{5}{6}$$