At a 6mph speed, it took Emily 60m
If she returns at a speed of 12mph, it'll take half the time so 30m.
The total of time she took to travel is
30m+60m=90m
the 6mph part represents 2/3 of the time
and the 12mph part reprensents 1/3 of the time
So then
(2/3•6)+(1/3•12)=8
So Emily has an average of 8mph for the entire ride