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To solve this problem, we need to calculate the percentage increase in heart rate as the running speed increases from 8 km/h to 12 km/h, using the graph provided.

### Step 1: Determine the heart rates at 8 km/h and 12 km/h.
- At **8 km/h**, the heart rate is approximately **130 bpm**.
- At **12 km/h**, the heart rate is approximately **150 bpm**.

### Step 2: Calculate the absolute increase in heart rate.
The increase in heart rate is:
\[
\text{Increase} = 150 \, \text{bpm} - 130 \, \text{bpm} = 20 \, \text{bpm}
\]

### Step 3: Calculate the percentage increase.
To find the percentage increase, use the formula:
\[
\text{Percentage increase} = \left( \frac{\text{Increase}}{\text{Initial value}} \right) \times 100
\]
Substituting the values:
\[
\text{Percentage increase} = \left( \frac{20}{130} \right) \times 100 \approx 15.38\%
\]

### Step 4: Approximate the answer.
The percentage increase is approximately **15.38%**, which is closest to **15%**.

However, looking at the available options, **20%** is the closest percentage, so the correct answer is **20%**.

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Would you like more details on the solution or to ask any questions?

### Related questions:
1. How do you calculate percentage change in general?
2. What other methods can you use to estimate increases from graphs?
3. How does heart rate typically change with running speed?
4. Can you use this method for different speed intervals?
5. How accurate does the graph need to be for these kinds of calculations?

**Tip:** Always use the exact values from a graph when possible, but rounding may sometimes be necessary to match answer choices.

The percentage increase in heart rate when running speed increased from 8 km/h to 12 km/h is closest to 10%, 20%, 25%, or 32%.

Learn how to calculate the percentage increase in heart rate as running speed increases from 8 km/h to 12 km/h using graph data. This problem involves key mathematical concepts such as percentage increase and interpreting graphs. Suitable for students in Grades 7-9.

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