On a plan that was scaled at 1 cm=40 metres, a surveyor measured the distance between two spots, and the result was 450 metres. But afterwards he realised he had been using a scale of 1 cm=20 m. then actual distance between the spots is:

Correct option is C

​$$\text{Given: Intended scale } = 1 \text{ cm } = 40 \text{ m}$$
$$\text{Wrong scale used } = 1 \text{ cm } = 20 \text{ m}$$
$$\text{Measured distance (with wrong scale)} = 450 \text{ m}$$
$$\text{Plan length} = \frac{450}{20} = 22.5 \text{ cm}$$
$$\text{Actual distance} = 22.5 \times 40 = 900 \text{ m}$$
$$\therefore \text{Actual distance between the two spots} = 900 \text{ m}$$​​​​​​​