Now to prove your answer is the smallest.
Last digit of result always is 1. We should prove it cannot be 1.
36k = 5m +1, left side is dividable by 4. Right side is not.

I was following the topic to see the solution to this seemingly simple but very interesting problem.
And very nice solution by plasmid. Even school kids could understand without any trouble.

I never try these riddles due to the language /and culture/ incompetency.
Could it be drug tablet. I am not sure if I understand the sentences accurately.