A Collection of Quant Riddles With (some) Answers

The quant riddles or logic or lateral puzzles 1-19 appear in the book 'Heard on The Street: Quantitative Questions from Wall Street Job Interviews' by Timothy Falcon Crack PhD available on his website the rest have been accumulated from the internet and emails that I receive. They are designed to help training for job or university interviews or just training your brain. The internet is littered with this kind of thing but the answers can be a little harder to find so I've thought about them all and answered all but one. I've added a 'Helpful Hint' button to each question. I've tried to keep the clues appropriate to the difficulty of the question but don't feel you have to read the whole thing, they add ideas incrementally. Questions 3 & 5 are probably the easiest and a good place to start. I've coloured them Red, Amber and Green to indicate Very Hard, Quite Hard and Not so Hard together with the indicator on the right. The difficulty is very subjective, it's only a guide, but they all offer something in teaching you the ideas and tricks used to solve them. So that's it good luck....

 You are given a set of scales and 90 coins. The scales are of the same type as above. You must pay \$100 every time you use the scales. The 90 coins appear to be identical. In fact, 89 of them are identical, and one is of a different weight. Your task is to identify the unusual coin and to discard it while minimizing the maximum possible cost of weighing (another task might be to minimizing the expected cost of weighing). What is your algorithm to complete this task? What is the most it can cost to identify the unusual coin?

© Nigel Coldwell 2013 - The questions on this site may be reproduced without further permission, I do not claim copyright over them. The answers are mine and may not be reproduced without my expressed prior consent. Please enquire using the link at the top of the page.

PayPal
I always think it's arrogant to add a donate button, but i've had as many as 1 emails requesting it. Don't feel you have to but if you want to then as little as £1 would pay for hosting for nearly a week. - nigel