$c$) $\dfrac{168.168 - 168.58}{110}$
$= \dfrac{168.(168-58)}{110}$
$= \dfrac{168.110}{110}$
$= 168$
$d$) $\dfrac{1}{30} + \dfrac{1}{42} + \dfrac{1}{56} + ... + \dfrac{1}{600}$
$= \dfrac{1}{5.6} + \dfrac{1}{6.7} + \dfrac{1}{7.8} + ... + \dfrac{1}{24.25}$
$= \dfrac{1}{5} - \dfrac{1}{6} + \dfrac{1}{6} - \dfrac{1}{7} + \dfrac{1}{7} - \dfrac{1}{8} + .... + \dfrac{1}{24} - \dfrac{1}{25}$
$= \dfrac{1}{5} - \dfrac{1}{25}$
$= \dfrac{4}{25}$
