The average speed of an object is computed as follows,
[tex]v=\frac{d}{t}[/tex] , where [tex]d[/tex] is the distance covered in the time interval of interest and [tex]t[/tex] is the time taken to cover the distance.
In this problem, the distance covered is
[tex]d= 3\frac{1}{6} km =\frac{19}{6} km.[/tex] as an equivalent fraction.
The time taken for the journey is
[tex]t=\frac{1}{4} hour.[/tex].
The avarage is speed is then,
[tex]v=\frac{d}{t}[/tex]
[tex]v=\frac{(19/6)km}{(1/4)h} =\frac{19\times 4 km}{6\times 1h} =\frac{38km}{3h} = 12.67km/h.[/tex]
