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A train travelling from Delhi to Ambala meets with an accident after $1$ hr. It is stopped for $\frac{1}{2}$ hr, after which it proceeds at fourth-fifth of its usual rate, arriving at Ambala at $2$ hr late. If the train has covered $80$ km more before the accident, it would have been just $1$ hr late. The usual speed of the train is :

- $20$ km/hr
- $30$ km/hr
- $40$ km/hr
- $50$ km/hr