### Brain Teasers

# A Card Puzzle

Probability
Probability puzzles require you to weigh all the possibilities and pick the most likely outcome.Probability

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?

I turn three facedown.

I turn the 2 and 8 faceup.

What is the probability of drawing a 1?

### Answer

1/8Facedown 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.

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