You take a three-question true or false quiz. You guess on all the questions. What is the probability that you will get a perfect score?

The probability that you will guess the first answer correctly  =  1/2  =  0.5
The probability that you will guess the second answer correctly  =  1/2  =  0.5
The probability that you will guess the third answer correctly  =  1/2  =  0.5

The probability that you will guess all three answers correctly is

                (0.5) x (0.5) x (0.5)  =  0.125  =  12.5% .

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Here are all the ways your answer sheet could wind up:
(R = a right answer.  W = a wrong answer.)

W W W 
W W R
W R W
W R R
R W W
R W R
R R W
R R R

Number of possible ways to score 0 right out of 3 questions:  1 way
Number of possible ways to score 1 right out of 3 questions:  3 ways
Number of possible ways to score 2 right out of 3 questions:  3 ways
Number of possible ways to score 3 right out of 3 questions:  1 way

Statistically, if your answers are all pure guesses, 
then on the average, the score you expect is

      (0 + 33-1/3 + 33-1/3 + 33-1/3 + 66-2/3 + 66-2/3 + 66-2/3 + 100) / 8

   =                  400 / 8

   =                      50%  .  

If you're happy with that score, great !  
Go to the movies the night before the test,
and start guessing.