QuestionThe table gives information about the times taken by 80 people to run a race.
Time taken ( $t$ minutes) Cumulative Frequency $$\begin{array}{ll} 50<t \leq 60 & 15 \\ 50<t \leq 70 & 31 \\ 50<t \leq 80 & 52 \\ 50<t \leq 90 & 66 \\ 50<t \leq 100 & 74 \\ 50<t \leq 110 & 80 \end{array}$$
This information is shown on the cumulative frequency graph below.
Use this graph to find an estimate for the interquartile range of the times taken.