Answer
She walked at 4 mph and jogged at 8 mph
Work Step by Step
We first find her average velocity:
$v = 20/4 = 5$ mph
We call j the jogging rate and w the walking rate. Recall, she jogs 40 percent of the time and walks 60 percent of the distance. Since her jogging rate is twice her walking rate, this means she jogs 25 percent of the time and walks 75 percent of the time. Thus:
$.25j + .75w = 5 \\ .25(2w) + .75w = 5 \\ w =4$
Since she jogs twice as fast, this means she jogs at 8 mph.
