Betina has 42 Kg of rice costing K640 per Kg. She wants to add rice costing K820 per Kg to make a mixture that costs K760 per Kg. How many kilograms of the K820 rice should she add?

• Mark 35: Appears 2 times
• Mark 36: Appears 3 times
• Mark 37: Appears 4 times
• Mark 38: Appears 1 time

• Draw a horizontal axis (x-axis) and label it "Marks".
• Draw a vertical axis (y-axis) and label it "Number of Students" or "Frequency".

• The maximum frequency is 4, so the y-axis should go up to at least 4 or 5. A scale of 1 unit per grid line is appropriate.

• For each mark, draw a bar whose height corresponds to its frequency.
• Ensure the bars have equal width and are equally spaced from each other.

Here is a description of the bar graph:

• X-axis (Marks): Label points at 35, 36, 37, 38.
• Y-axis (Number of Students): Label points at 0, 1, 2, 3, 4.
• Bar for Mark 35: Height up to 2.
• Bar for Mark 36: Height up to 3.
• Bar for Mark 37: Height up to 4.
• Bar for Mark 38: Height up to 1.

There are 10 marks, so $n = 10$.

Since $n$ is an even number, the median is the average of the two middle values. These are the $(\frac{n}{2})^{\text{th}}$ and $(\frac{n}{2} + 1)^{\text{th}}$ values.

• $\frac{10}{2} = 5^{\text{th}}$ value
• $\frac{10}{2} + 1 = 6^{\text{th}}$ value

The 5th value is 36.
The 6th value is 37.

The median mark is $\boxed{36.5}$.