<PRE><B>Question 579987</B>

Entire round trip journey = 800 miles

solve for time:

t = 800 / 96 mph = 8 1/3 hours needs to be total time

already used:

t = 400 / 80 = 5 hours

3 1/3 hours left to go 400 miles back, solve for speed:

v = 400 / (3 1/3) = 120 mph

its not a simple average, since the train spends different times at each speed

CHECK:

800 miles, total time 8 1/3 hours 

average mph = miles/hours = 800 / (8 1/3) = 96</PRE>