Answer
$s=19.1~min$
Work Step by Step
$x_1=First~Class~Midpoint=5$
$x_2=Second~Class~Midpoint=15$
$x_3=Third~Class~Midpoint=25$
$x_4=Fourth~Class~Midpoint=35$
$x_5=Fifth~Class~Midpoint=45$
$x_6=Sixth~Class~Midpoint=55$
$x_7=Seventh~Class~Midpoint=65$
$x_8=Eighth~Class~Midpoint=75$
$x_9=Nineth~Class~Midpoint=85$
$x ̅=27.7$ (item (a))
$s=\sqrt {\frac{Σ(x_i-x ̅)^2f_i}{Σfi-1}}=\sqrt {\frac{(5-27.7)^2\times125+(15-27.7)^2\times271+(25-27.7)^2\times186+(35-27.7)^2\times121+(45-27.7)^2\times54+(55-27.7)^2\times62+(65-27.7)^2\times43+(75-27.7)^2\times20+(85-27.7)^2\times13}{895-1}}=19.1$
