$13,14,15,16,\underbrace {17,18}_{{Q_2} = 17/5},18,19,20,20$
$\bar X = \frac{{\sum\nolimits_{i = 1}^n {{x_i}} }}{n} = \frac{{170}}{{10}} = 17$
$\sigma = \sqrt {\frac{{\sum\nolimits_{i = 1}^n {{{(\bar X - {x_i})}^2}} }}{n}} $
$\sigma = \sqrt {\frac{{9 + 4 + 0 + 1 + 4 + 9 + 9 + 16 + 1 + 1}}{{10}}} = \sqrt {5/4} $