Plot the scatter diagram.

a) To plot a scatter diagram, you need to draw two perpendicular axes on graph paper. The horizontal axis (x-axis) represents Mathematics marks, and the vertical axis (y-axis) represents Statistics marks.
For each pair of marks $(x, y)$ from the table, plot a point on the graph.
The points to plot are:

$$(11, 26), (20, 23), (23, 35), (42, 46), (48, 44), (50, 50), (57, 50), (64, 58), (80, 68), (90, 70)$$

The scatter diagram will show a general upward trend, indicating a positive correlation between Mathematics and Statistics marks.

b) After plotting all the points, draw a straight line that best represents the trend of the data. This line, called the line of best fit, should be drawn such that:

• It passes through the general center of the points.
• Approximately an equal number of points lie above and below the line.
• It follows the overall direction of the points. This line does not necessarily have to pass through any of the plotted points.

c) To estimate a student's mark in Statistics if their mark in Mathematics is 40, you would use the line of best fit drawn in Step 2.

1. Locate $x=40$ on the Mathematics (horizontal) axis.
2. Draw a vertical line from $x=40$ upwards until it intersects your line of best fit.
3. From the intersection point on the line of best fit, draw a horizontal line to the Statistics (vertical) axis.
4. Read the value where this horizontal line intersects the Statistics axis. This value is the estimated mark.

Based on the general trend of the data, if a student scores 40 in Mathematics, their estimated mark in Statistics would be approximately $\boxed{42}$.