The answer is: average speed is 7.5 miles per hour.
t = 4 min ÷ 60 min/h.
t = 0,067 h; time Shyla ran the last half mile.
d = 0.5 m; distance Shayla run in four minutes.
v = d ÷ t.
v = 0.5 m ÷ 0.067 h.
v = 7.5 mph; average speed in miles per hour.
The speed (scalar quantity) of an object is the magnitude of its velocity.