A student takes a 20-question, multiple-choice quiz with 5 choices for each question, find the probability of guessing at least 15 correct out of the 20. Would you consider this event likely or unlikely to occur?
On your TI-83 or TI-84 calculator, type in
1-binomcdf(20,1/5,14)
and press ENTER.
You'll see which means
or
which is about 1 chance in 5.5 million.
So it's extremely unlikely.
Edwin