Brain Teasers Can Require To Think As A Management Consultant

Two fuses need exactly an hour to burn completely once lit. The combustion is not homogeneous along their length. For example, if half the length is burned, this does not mean that half an hour is past. How can you measure 45 minutes with these fuses?
Here we have a historical brainteaser, which needs to be handled as a managmeent consultant with extreme caution
Firstly, if you aim at McKinsey and if you have already trained on the PST, you know that the correct answer for such a statement is not “enough data to respond.” Why?


brain teaser mèches


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