You are given a task of locating one faulty coin out of 99 identical in appearance coins i.e. 98 of equal weight and one of a lesser weight; and to achieve it within seven weighings, using a balance scale.

Sounds familiar and easy (3⁷>99)?

Familiar? Yes.

Easy? Not quite!

How can you accomplish it if no coin may be weighed more than twice ?

Bonus task: After devising the requested procedure please generalize:
How many coins (n-1 normal and one lighter) can be resolved within k weighings?

(In reply to Any thoughts on this one? by tomarken)

What would you prefer?

a. a solution

b. a hint (like 1st set of marbles }

c. the methodology

d. the story of my error (BTW 96+1 is solvable)

I will gladly cater to your choice, believing that each of the above

will lead you to correct solution. 

Posted by Ady TZIDON on 2014-07-02 02:46:21