Question 204073
velocity  = distance/time,,,or  t=d/v
.
,total  time  =  d/x +d/y =(dy+dx)/xy = d(x+y)/xy
.
total  distance  =2d
.
velocity  total  = total  distance / total time = 2d / {d(x+y)}/xy = 2xy/x+y
.
check,,let  x=50, y=60, d=100  (all  arbitrary)
.
vel = 2(50)(60) / (50 +60) = 6000/110=54.55  mph
.
BIG  NOTE,  it  is  NOT (50 +60) /2 = 55,,,,or  (x+y)/2
.
avg  vel   = distance/time,,,,,,200 / {100/50 +100/60} = 54.55