Accidentally, as written, the problem does not have sufficient information to give a proper answer without making assumptions about information not actually given in the problem.
For example, did both buses return to the depot AFTER they โcompletedโ their routes, or did one or both of them go somewhere else after their respective route completions? (e.g. did one or both go to a gas station to refill the gas tank, or perhaps to a repair shop, etc.)
To answer the question as stated we would have to โassumeโ (and we all know what that means) that they both returned to the depot after their route AND remained there until at least the other one got back.
Now, IF that assumption is correct, then they would both be at the depot again at 8:35 am.
If thats not right then its 10:55