1. **State the problem:** We need to find the average speed of an aeroplane that travelled 1275 miles in 3 hours. 2. **Formula:** Average speed is given by the formula: $$\text{Average speed} = \frac{\text{Distance}}{\text{Time}}$$ 3. **Substitute the values:** $$\text{Average speed} = \frac{1275}{3}$$ 4. **Calculate:** $$\text{Average speed} = 425$$ 5. **Interpretation:** The aeroplane's average speed is 425 miles per hour (mph).