What is the probability of a student guessing randomly on a test and getting at least 8 out of 10 true and false answers correct?
1 Answer
Feb 21, 2017
In total there are
Explanation:
"At least 8 correct" translates to "at most 2 wrong".
0 wrong: can only be done in 1 way
1 wrong: can be done in 10 ways (any of ten questions)
2 wrong: the first wrong may be in 10 places, the second in the nine places left. But since the order of the wrongs doesn't matter, we halve that. (because number2+5 wrong is the same as number5+2 wrong). So in all: 2 wrong in (10x9):2=45 ways.
Total number of ways in which the student may have at most 2 wrongs (=at least 8 right) is:
On a total of