Claire’s speed = 6 km/hr
Distance that can be covered in 20 mins (one-third hour) = 1/3 x 6 = 2 km
So if Claire had left 20 mins she would have covered 2 km more. Since she was 3 km away from the library, she would have been 1 km away if she had left on time.
Since Claire and Charles usually meet at the library, Charles has to cover 3 km whilst Claire would have to cover 1 km. So Charles’ speed is three times that of Claire.
Charles’ speed = 3 x 6 = 18 km/hr