A bike moving at a speed of 45 km/hr covers certain distance in 4 hours. If its speed is reduced by 20 km/hr, in what time it will cover the same distance?

Initial speed of the bike = 45 km/hr

Time taken at this speed = 4 hours

New speed = 45 km/hr - 20 km/hr = 25 km/hr

​$$\text{Distance} = \text{Speed} \times \text{Time}\\ \ \\\text{Time} = \frac{\text{Distance}}{\text{New Speed}}​$$​​

Total distance in first condition = $$45\times4= 180\ km$$ 

Thus, correct option is (b).