Mental Teacher

Once a teacher asked his Students that if a Train is traveling 5 km per hour, Then tell me What will be my age.
Students could not understand this; they requested to please repeat the statement.
Teacher Again Said. That if a Train is Traveling at 5km per hour what is my age.
All were confused that what sense it makes.There is no correlation between the train speed and his age.After sometime..
One of the smart students raised his hand and said Sir Should I reply.
The Teacher: Oh, Yes Please.
Student. Sir your age is fifty years
Teacher. Good, very good that is true
But how you calculated that,
Student. Sir my brother is sick, he talks in a way that makes very poor sense, people feel difficulty in understanding his talking ,when we took him to the doctor, he diagnosed him a case of mental illness, and labeled him “HALF MENTAL”.
His age is 25 Years, as your talking makes absolutely no sense, so I thought your age must me double to my brother’s age. As he is 25 so you must be at least 50 years.

Good one .

Thanks Zubair