Выполните действие: а) 2/9 + 5/15 б) 17/20 - 5/12 в) 11/12 - - (2/3 + 1/4)

Решите задачу:

$\frac{2}{9} + \frac{5}{15} = \frac{30}{135} + \frac{40}{135} = \frac{75}{135} = \frac{5}{9}$
$\frac{17}{20} - \frac{5}{12} = \frac{51-25}{60} = \frac{26}{60} = \frac{13}{30}$
$\frac{11}{12} - ( \frac{2}{3} + \frac{1}{4} ) = \frac{11}{12} - ( \frac{8}{12} + \frac{3}{12} ) = \frac{11}{12} - \frac{11}{12} = 0$