Đáp án:
$B = 3\dfrac{1}{5} - 2\dfrac{7}{8} - 30$%
$ = \dfrac{16}{5} - \dfrac{23}{8} - \dfrac{30}{100}$
$ = \dfrac{128}{40} - \dfrac{115}{40} - \dfrac{30}{100}$
$ = \dfrac{13}{40} - \dfrac{30}{100}$
$ = \dfrac{65}{200} - \dfrac{60}{200}$
$ = \dfrac{5}{200}$
$= \dfrac{1}{40}$