$(1-\dfrac{1}{15}) \times (1- \dfrac{1}{21}) \times (1 - \dfrac{1}{28}) \times ..... \times (1 - \dfrac{1}{210}$
$= \dfrac{14}{15} \times \dfrac{20}{21} \times \dfrac{27}{28} \times ..... \times \dfrac{209}{210}$
$= \dfrac{28}{30} \times \dfrac{40}{42} \times \dfrac{54}{56} \times .... \times \dfrac{418}{420}$
$= \dfrac{4\times7}{5\times6} \times \dfrac{5\times8}{6\times7} \times \dfrac{6\times9}{7\times8} \times .... \times \dfrac{19\times22}{20\times21}$
$= \dfrac{4\times5\times6\times.......\times19}{5\times6\times7\times.....\times20} \times \dfrac{7\times8\times9\times......\times22}{6\times7\times8\times.....\times 21}$
$= \dfrac{1}{5} \times \dfrac{11}{3}$
$= \dfrac{11}{15}$
