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Senior Applications Developer Interview

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Deutsche Bank

There are 10 stacks of 10 coins each. 9 of the stacks contain coins that weigh 1g each. The other stack contains coins of 2g each. The coins look the same. We have a scale that we can get a measurement of grams from, not a balance. We can use the scale exactly once to weigh anything here from a single coin to all of them. How can we determine which stack is the 2g coins?

Tags:logic, algorithm, math

Interview Answers

4 Answers

10

Weigh these together: 10 coins from stack 1, 9 from stack 2, etc ending with 1 from stack 10. The weight of these will tell you which stack has the 2g coins. Ex: if it's 1st stack: 65g, 2nd: 64g, 10th: 54g

Anonymous on

2

A more eloquent answer would be: Weigh together 1 coin from stack 1, 2 coins from stack2, 3 coins from stack 3, etc. Subtract 55 from that total weight to get the number of the stack with the 2g coins.

Joe on

1

Excellent question!

Jatin Rai on

0

Binary sort 1. Use single coin (I'd drop my stacks *s*) from each stack. Mark them by stack. 2. Place 5 each on both scales; note lower scale. 3. Place 2 coins on each scale, keep one aside; note scale. 4. If scales are at same level the coin NOT on the scale indicates the stack with heavier coins. Done. 5. If one scale is lower place 1 coin on each scale, note lower, identify stack. Done.

Doug Dodge on

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