Answer
$s=14.72~min$
Work Step by Step
$x_1=First~Class~Midpoint=45$
$x_2=Second~Class~Midpoint=55$
$x_3=Third~Class~Midpoint=65$
$x_4=Fourth~Class~Midpoint=75$
$x_5=Fifth~Class~Midpoint=85$
$x_6=Sixth~Class~Midpoint=95$
$x_7=Seventh~Class~Midpoint=105$
$s=\sqrt {\frac{Σ(x_i-x ̅)^2f_i}{Σfi-1}}=\sqrt {\frac{(45-74.85)^2\times8+(55-74.85)^2\times44+(65-74.85)^2\times23+(75-74.85)^2\times6+(85-74.85)^2\times107+(95-74.85)^2\times11+(105-74.85)^2\times1}{200-1}}=14.72$
