1. **State the problem:** Hamish needs foals weighing between $49.5 - 2 = 47.5$ kg and $49.5 + 2 = 51.5$ kg for his research. 2. **Identify the weights that qualify:** Foals must weigh between $47.5$ kg and $51.5$ kg inclusive. 3. **List the foal weights:** $49.5, 46.9, 47.9, 51.6, 49.7, 49.4, 51.5, 47.0, 48.6, 50.6, 51.8, 48.8, 48.0, 47.5, 50.0, 51.4, 47.4, 51.0, 49.1, 47.6$ 4. **Determine which foals are outside the range:** - Below $47.5$: $46.9, 47.0, 47.4$ - Above $51.5$: $51.6, 51.8$ 5. **Count foals outside the range:** Number outside = $3 + 2 = 5$ 6. **Calculate percentage of foals Hamish cannot use:** Total foals = $20$ $$\text{Percentage} = \frac{5}{20} \times 100 = 25\%$$ **Final answer:** Hamish cannot use $25\%$ of the foals.