24 pupils had just written a math test, sixth grade, percentage calculation, something even most adults could still manage.
Some pupils had been worried, some frustrated, some confident in their abilities and some had been very confused.
Had the teacher been telepathic, he would have noticed it tended to be the same people, but he wasn't.
And so he didn't notice that his best student was despairing over the last problem.
"An AIDS-test has a 95% accuracy. 320 people have been tested negative. How many of them are stil HIV-positive?", she had read out to herself, whispered and almost inaudible.
She hadn't been able to answer.
It was possible that none of them were infected, after all, a 95 percent accuracy was pretty good. On the other hand it could have gone by the numbers and 16 people got a wrong diagnosis. Or maybe it was an extremely special case and all 320 people were really infected.
There had been no right answer, simply because the accuracy didn't give a fact, just a probability, but they hadn't done the theory of probabilities yet, she didn't know how to calculate that kind of thing properly.
She had pondered over it, something the many scratched out, garbled solution statements showed.
In the end she had declared the entire row of solution statements hopeless and written down a single phrase under it, one she had read countless times in internet IQ-tests: "The solution is not explicitly deductable from the problem."
She had smiled, having given the right answer and all, at least in the light of all her mathematical knowledge.
The teacher was shaking his head quietly and sighed as he marked the sentence with a red scribble. Because it wasn't recognizable as such he always had to tell his new pupils that it was a W. W for wrong, failure.
And the problem had been such an easy one.