Marvell Semiconductor Interview Question: You have seven stones and a w... | Glassdoor

Interview Question

Design Verification & Test Engineer Interview(Student Candidate) Santa Clara, CA

You have seven stones and a weighing scale. Six of the

  stones are equal in weight and one is lighter. How will you figure out which one is lighter ? Minimum tries required to do so ?

Interview Answer

5 Answers


Trail 1: At random weigh two stones vs. two stones (3 sitting on the side)
             A: Of the 4 on the scales if one side weighs more then the other then weigh one on each side (since one of them must be heavier)
             B. If the 2 vs 2 are equal then at random weigh 2 (one on each side) of the three left on the side. If they are the same then the 3rd one that never got weighed is the heaviest.

Simple case of process of elimination by grouping (Divide and Test)

RKPATEL on Oct 5, 2010

I would first weigh in one stone, say stone 1, and assume the weight is say 2 lbs(try 1). Then separate the 6 remaining stones into 2 piles, 2,3,4 and 5,6,7. Weigh in either 2,3,4 or 5,6,7, it doesn't matter. Say 2,3,4, if the sum of these 3 is 6 then the lighter stone has to be in the 5,6,7(try 2). Weigh in 5,6, if the sum of the two is 4 then the lighter is stone 7(try3). If sum is less than 4 then weigh in either 5 or 6 to find out. So, the maximum number of tries is 4 and least is 3.

Anonymous on Oct 5, 2010

Needs two weighing at most:
1. Put {1, 2, 3} on LHS and {4, 5, 6} on RHS.
2. If LHS and RHS are equal 7 is the lighter one.
    else discard heavier of previously weighed group.
Now we have a group 3 stones left. Lets call them A, B, C.
3. Put A on LHS and B on RHS.
4. If LHS and RHS are equal C is the lighter one.
    else lighter or LHS or RHS is the lighter one.

Abe on Mar 12, 2011

Two tries.
1st try: 3 : 3, 7th is fake if equal; otherwise,
2nd try: 1:1 picked from the light triple in 1st try. the lighter one is fake if any, the third one fake otherwise.

Chenpp on Aug 23, 2011

If the stones are made of the same material... they likely have the same density... therefor whichever one looks the smallest, will weigh the least...

there... 1 step... just look for the one that is the smallest.

Bryan on Apr 30, 2014

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