a) $$14\frac{9}{13} - 1\frac{5}{13} + 12\frac{11}{13} = (14-1+12) + (\frac{9}{13} - \frac{5}{13} + \frac{11}{13}) = 25 + \frac{15}{13} = 25 + 1\frac{2}{13} = 26\frac{2}{13}$$
б) $$7\frac{24}{25} - 3\frac{12}{25} - 1\frac{7}{25} = (7-3-1) + (\frac{24}{25} - \frac{12}{25} - \frac{7}{25}) = 3 + \frac{5}{25} = 3 + \frac{1}{5} = 3\frac{1}{5}$$
Ответ: a) $$26\frac{2}{13}$$; б) $$3\frac{1}{5}$$