First of all, because nothing in the statement specifies that you have matches!
Then we get to the heart of the matter. We should beware of assumptions.
- Here is the ‘classic’ solution which is not, in this form, acceptable in a consulting interview.
- We light one of the fuses from both ends, and the other by one end only: the first is consumed in half an hour, then at this moment we turn on the other end of the second fuse: it will be consumed in 15 minutes. Then, since the initial lighting 45 minutes will have passed.
What’s wrong? It lacks a hypothesis!
Just take 2 minutes before reading the reasonning below…
Imagine for some reason that the combustion requests to “take into consideration that the fuse may not burn evenly. It may take 45 minutes to consume the first 10% and then 15 minutes to burn the rest.” Then in such a case, after 30 minutes the lit fuse on both sides is far from being completely consumed.
- All this is to illustrate that we must be careful with the assumption we have added to the statement, ensuring that a given fuse is entirely burned in 30 minutes if we light it from both sides — which is the assumption of departure.
- It is with this type of thinking that you score points, even on an issue discussed for many years!
Other Resources On Brain Teasers
|Links of interest|| - Introduction To Brain Teasers
- Brain Teasers From Wall Street
|Videos on this topic||- Classical Brain Teasers|
|Related concepts||- brain teasers finance
- brain teasers questions
- brain teasers consulting
- brain teasers interview