в) \frac{9}{10}m-\frac{2}{15}m-\frac{3}{5}m;

в) Приведем дроби к общему знаменателю:

$$\frac{9}{10}m-\frac{2}{15}m-\frac{3}{5}m = \frac{9}{10}^{\scriptsize \,3}m-\frac{2}{15}^{\scriptsize \,2}m-\frac{3}{5}^{\scriptsize \,6}m = \frac{9\cdot3}{10\cdot3}m-\frac{2\cdot2}{15\cdot2}m-\frac{3\cdot6}{5\cdot6}m = \frac{27}{30}m-\frac{4}{30}m-\frac{18}{30}m = \frac{27-4-18}{30}m = \frac{5}{30}m = \frac{5}{5\cdot6}m = \frac{\cancel{5}}{\cancel{5}\cdot6}m = \frac{1}{6}m.$$

Ответ: $$\frac{1}{6}m$$.