Answer:
He must start his journey by 11 AM
Step-by-step explanation:
Speed of Toshi while walking up the mountains is 1.5 kilometres per hour
Speed of Toshi while walking down the mountains = 2 * Speed of Toshi while walking up the mountains = 3 kilometres per hour
Time taken to travel 9 Km in the upward direction by Toshi = 9 Km/1.5 kilometres per hour = 6 hours
Time taken to travel 9 Km in the downward direction by Toshi = 9 Km/3 kilometres per hour = 3 hours
He will take total 6 hours + 3 hours = 9 hours to travel 18 Km distance
Thus, He must start his journey by 11 AM