Given: Odyssey runs from the Trojan Horse, which does not move, to the gates of Troy and back to the Trojan Horse by the same path.
Asked: Was his average speed less than 20 kilometres per hour for the entire journey ?
Let the distance between Trojan horse and Troy be D km and time taken from Trojan horse to Troy be t1 and time taken back to Trojan horse from Troy be t2.
Average speed for the entire journey = 2D/(t1+t2)
v1 = D/t1
v2 = D/t2
(1) Odyssey's average speed from the Trojan Horse to the gates of Troy was greater than 40 kilometres per hour.
v1 = D/t1 > 40 kmph
D > 40t1
Average speed for the entire journey > 80t1/(t1+t2)
If t2 = 0; Average speed for the entire journey > 80 kmh
But if t2 is infinite; Average speed for the entire journey = 0 kmh
NOT SUFFICIENT
(2) Odyssey's average speed from the gates of Troy to the Trojan Horse was less than 10 kilometres per hour.
v2 = D/t2 < 10 kmph
D < 10 t2
Average speed for the entire journey < 20t2/(t1+t2)
If t1=0; Average speed for the entire journey < 20 kmph
But if t2 is infinite; Average speed for the entire journey = 0 kmph
In both extreme case, Average speed for the entire journey < 20 kmph
SUFFICIENT
IMO B