Question 1042801
A true/false test has 4 questions. What is the probability of getting at least 3 right by guessing the answers randomly?
<pre>Probability of getting at least 3 means the probability of getting 3 or the probability of getting 4 correct. 
Use the BINOMIAL PROBABILITY formula: {{{" "[n]C[s] * p^s * q^(n - s)}}}
Probability of getting 3 correct: {{{matrix(1,3, " "[4]C[3] * (.5)^3 * (.5)^(4 - 3) = 1/4, or, .25)}}} 
Probability of getting 4 correct: {{{matrix(1,3, " "[4]C[4] * (.5)^4 * (.5)^(4 - 4) = 1/16, or, .0625)}}} 
Probability of getting at least 3 correct (probability of getting 3 or 4 correct): {{{highlight_green(matrix(1,3, .25 + .0625, or, highlight(.3125)))}}}
<b><u>OR</b></u>
Use the BINOMCDF feature on a calculator (e.g. TI-83/84). Enter: binomcdf(4,.5,1). You should also get {{{highlight(.3125)}}}
<b><u>OR</b></u>
Use MS Excel. Enter: "=1-binomdist(3-1,4,.5,1." You should also get the above answer.