$M = \dfrac{1}{24}.(\dfrac{27}{3.5}+\dfrac{27}{5.7}+...+\dfrac{27}{25.27})$
$ = \dfrac{27}{24}.(\dfrac{27}{3.5}+\dfrac{27}{5.7}+...+\dfrac{27}{25.27})$
$ = \dfrac{27}{48}.(\dfrac{2}{3.5}+\dfrac{2}{5.7}+...+\dfrac{2}{25.27})$
$ = \dfrac{9}{16}.(\dfrac{1}{3}-\dfrac{1}{27})$
$ = \dfrac{9}{16}.\dfrac{8}{27}$
$ = \dfrac{1}{6}$
Vậy $M = \dfrac{1}{6}$