Data Representation: Frequency Tables and Cumulative Curves

The cumulative frequency curve below shows the marks obtained by \(250\) pupils in a particular examination. The maximum mark is \(80\).

Cumulative frequency curve of examination marks

From the graph, estimate

1. the median of this distribution,
2. the interquartile range,
3. the \(80^{\mathrm{th}}\) percentile,
4. the minimum mark such that a pupil can be in the top \(20\%\).