Tính: 

        a) $\frac{1}{3}$  + $\frac{1}{4}$  = ......................                                   b)  $\frac{3}{5}$   - $\frac{2}{7}$     = .......................

                       = ......................                                                          = .......................

        c) $\frac{5}{7}$  : $\frac{2}{3}$  = ......................                                  d)   $\frac{3}{5}\times \frac{2}{7}$   = ......................

                       = ......................                                                     = .......................