When Sourav increases his speed from 20 Km/hr to 25 Km/hr, he takes one hour less than the usual time

When Sourav increases his speed from 20 Km/hr to 25 Km/hr, he takes one hour less than the usual time to cover a certain distance. What is the distance usually covered by him?

1. 125 km
2. 100 km
3. 80 km
4. 120 km