Thực hiện các phép tính (tính hợp lí nếu có thể): (a) 7/15 + 1/3 − 3/5 . (b) 12/13 : ( 3/26 − 2/13 ) .

a) \[\frac{7}{{15}} + \frac{1}{3} - \frac{3}{5} = \frac{7}{{15}} + \frac{5}{{15}} - \frac{9}{{15}} = \frac{3}{{15}} = \frac{1}{5}\].

b) \[\frac{{12}}{{13}}:\left( {\frac{3}{{26}} - \frac{2}{{13}}} \right) = \frac{{12}}{{13}}:\left( {\frac{3}{{26}} - \frac{4}{{26}}} \right) = \frac{{12}}{{13}}:\frac{{ - 1}}{{26}} = \frac{{12}}{{13}}.\frac{{26}}{{ - 1}} = - 24\].

c) \[\frac{{ - 3}}{5}.\frac{5}{7} + \frac{{ - 3}}{5}.\frac{3}{7} + \frac{{ - 3}}{5}.\frac{6}{7}\]\[ = \frac{{ - 3}}{5}.\left( {\frac{5}{7} + \frac{3}{7} + \frac{6}{7}} \right)\]\[ = \frac{{ - 3}}{5}.2 = \frac{{ - 6}}{5}\].