Myths Busted

I have exams approaching , but I am still in “jolly good” mood. My professor just announced an open book exam for a subject and we were all freaking out. I had written an open-book exam previously and I was quite happy with my results. I concluded that secret to open-book exam’s success is intuitive thinking. But my prof today busted this myth. He gave a task. He would ask us a question based on basic arithmetic and we need to answer it in less than 2 seconds

Question: Boy purchased a pen and eraser for Rs. 11 and the difference between their costs are Rs.10. What is the cost of the eraser?
While I was still thinking the answer, one of my classmates answers Rs.1 in less than a second. For a tiny moment, we all thought the answer is right.
This all happened in a matter of few seconds.

Then immediately, he asked us the same question but this time, he gave us sufficient time to think about the answer. And eventually, we came up with the correct answer. ( Rs 0.5 is the cost of the eraser).

So, our initial answer was based on intuition, which may or not be correct always. But it looks correct most of the times (Trap !). So in open book exams, one needs to think deeper and solve rather than surfing superficially for an approximate answer.

I hope I do well in my Friday’s exam. Friday 22nd marks the end of exams.
I am counting days for my travel to home.