5. 计算：
(1)$(+84)÷(-21)$；
(2)$(-1\frac{5}{8})÷(+2\frac{3}{5})$；
(3)$16÷(-\frac{4}{3})÷(-\frac{9}{8})$；
(4)$(-3)÷(-\frac{2}{5})÷(-\frac{1}{4})$；
(5)$(-0.75)÷(-3)÷\frac{5}{4}$；
(6)$(-5)÷[(-\frac{3}{10})÷(-\frac{1}{2})]$。

(1)
$(+84)÷(-21)=-(84÷21)= - 4$
(2)
$-1\frac{5}{8}=-\frac{13}{8}$，$+2\frac{3}{5}=\frac{13}{5}$
$(-1\frac{5}{8})÷(+2\frac{3}{5})=(-\frac{13}{8})÷\frac{13}{5}=-\frac{13}{8}×\frac{5}{13}=-\frac{5}{8}$
(3)
$16÷(-\frac{4}{3})÷(-\frac{9}{8})=16×(-\frac{3}{4})×(-\frac{8}{9})=-12×(-\frac{8}{9})=\frac{32}{3}$
(4)
$(-3)÷(-\frac{2}{5})÷(-\frac{1}{4})=(-3)×(-\frac{5}{2})×(-4)=\frac{15}{2}×(-4)= - 30$
(5)
$(-0.75)÷(-3)÷\frac{5}{4}=(-\frac{3}{4})×(-\frac{1}{3})×\frac{4}{5}=\frac{1}{4}×\frac{4}{5}=\frac{1}{5}$
(6)
$(-\frac{3}{10})÷(-\frac{1}{2})=\frac{3}{5}$
$(-5)÷[(-\frac{3}{10})÷(-\frac{1}{2})]=(-5)÷\frac{3}{5}=(-5)×\frac{5}{3}=-\frac{25}{3}$