Teaser Of The Day No. 33

[Carl]

I have ten cards 1-10.
I turn three facedown.
I turn the 2 and 8 faceup.

What is the probability of drawing a 1 ?


Answer will be provided next Thursday evening after 21:00.

[Carl]

ANSWER

1/8

Facedown cards do not effect the probability.

For example:

I have three cards and I turn one facedown.

There is a 1/3 chance that the 1 card is facedown...
1/3(1/1)+2/3(0/2)=1/3

...and a 2/3 chance that it isn't:
1/3(0/1)+2/3(1/2)=1/3

Both have the same probability.