You should always change....................
Lets assume there were ten doors and behind nine there is nothing..................behind one there is a car.
You pick number six.........................you have a one in ten chance of being correct.
Now the host (Monty Hall) removes eight leaving number four.......................he cannot remove the door that has the car behind it.

Now your number six is still only a one in ten chance of being right and the other door is a NINE in ten chance of being right.

It is not fifty/fifty although there are only two cards left.

So which door is is probably best to open? the one offered by the host.

Magicman