Average Speed = Total distance/ total time

Given, speed from Port Wakefield to Port Adelaide = 12mph and return takes twice as long (or)
Speed from Port Adelaide to Port Wakefield = 6mph (half the speed)

Let the distance be '12' miles

So, onward journey would take 1 hr and return 2 hrs, total = 3hrs and distance (2*12 = 24)

Average Speed = 24/3 = 8mph. C IMO

speed for the going journey =12;
return journey takes double time, mean speed becomes half, so speed here is =6;
so average speed when distance for going and returning journey is same(2ab)/a+b;
a= speed for going journey;( in this case = 12)
b= speed for returning journey;( in this case = 6)
hence average speed becomes = 2(12*6)/18 = 8;