A large study was done on sleep habits of preteens and teenagers. Each individual was asked about the number of hours of sleep they normally get on an average night.

$$ \scriptsize \begin{array} {|c||c|c|c|c|} \hline & \lt \text{6 hours} & \text{6 to 8 hours} & \gt \text{8 hours} & \text{Total students} \\ \hline \text{Males thirteen or younger} & 6{,}143 & 10{,}362 & 3{,}411 & 19{,}916 \\ \hline \text{Females thirteen or younger} & 4{,}722 & 13{,}152 & 5{,}151 & 23{,}025 \\ \hline \text{Males older than thirteen} & 1{,}747 & 8{,}122 & 1{,}720 & 11{,}589 \\ \hline \text{Females older than thirteen} & 4{,}555 & 13{,}112 & 7{,}921 & 25{,}588 \\ \hline \text{Total} & 17{,}167 & 44{,}748 & 18{,}201 & 80{,}118 \\ \hline \end{array} $$

Based on the table, what is the approximate probability that a preteen (thirteen years or younger) would normally sleep for 8 or less hours?

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We want to find the following:

$$ \frac{\colorbox{aqua}{\text{a preteen that sleeps for 8 or less hours}}}{\colorbox{yellow}{\text{a preteen}}} $$ $$ \begin{array} {|c||c|c|c|c|} \hline & \lt \text{6 hours} & \text{6 to 8 hours} & \gt \text{8 hours} & \text{Total students} \\ \hline \text{Males thirteen or younger} & \colorbox{aqua}{$6{,}143$} & \colorbox{aqua}{$10{,}362$} & 3{,}411 & \colorbox{yellow}{$19{,}916$} \\ \hline \text{Females thirteen or younger} & \colorbox{aqua}{$4{,}722$} & \colorbox{aqua}{$13{,}152$} & 5{,}151 & \colorbox{yellow}{$23{,}025$} \\ \hline \text{Males older than thirteen} & 1{,}747 & 8{,}122 & 1{,}720 & 11{,}589 \\ \hline \text{Females older than thirteen} & 4{,}555 & 13{,}112 & 7{,}921 & 25{,}588 \\ \hline \text{Total} & 17{,}167 & 44{,}748 & 18{,}201 & 80{,}118 \\ \hline \end{array} $$ Note that because the question does not specify, we include both males and females. Similarly, 8 or less hours include both the first two columns. $$ \frac{6{,}143+10{,}362+4{,}722+13{,}152}{19{,}916+23{,}025}$$ $$ = \frac{34{,}379}{42{,}941} $$ $$ \approx \boxed{0.80} $$