Woman Gets Driver's License After 950 Tries

mr crypto writes "From the BBC News article, 'A South Korean woman is celebrating after passing the written exam for a driving license — on her 950th attempt. After four years of trying, 68-year-old Cha Sa-soon finally managed to secure the 60 out of 100 points needed to pass the test.' Maybe some things should just not be."

Comments

Re:What are the odds

[SpottedKuh]

With that many tries she should have been able to choose random answers and still pass the test with a 60% a long time ago

Nope, not if she chooses randomly for each question, each time she writes the test. Assume that for each of the 100 questions, there are four possible answers, only one of which is correct. So, you have a 25% chance of getting each question correct and a 75% chance of getting each question wrong, each time a random test-taker writes the exam.

The odds of passing a test are: the sum from c=60 to 100 of getting c questions correct.

The chances of getting c questions correct is (100 choose c) * (1/4)^c * (3/4)^(100-c).

Summing up from c=60 to 100, you are left with a probability of 0.1326834674 * 10^(-12) of passing the test, if you take it truly at random each time you write it. Basically, if you wrote the test truly at random, you would expect to have to take the test 7,536,733,998,027 times before passing. So, really, she did quite well ;)