Boxes of Money

Question: You are given b boxes and n dollar bills. The money has to be sealed in the b boxes in a way such that without thereafter opening a box, you can give someone a requested whole amount of dollars from 0 to n. How should b be related to n for this to happen?

Answer: Stumped? Let’s think of an example to approach this problem.

Say we have $100. A good approach to distributing $100 would be the binary number system. So you’d have $1, $2, $4, $8, $16, $32 in the first six boxes. We can’t fill the next box with $64 dollars because we are only left with $37 dollars (from a total of $100). So we’d have to put $37 in the seventh box. To supply any requested amount, we’d have to use a combination of these boxes.

To find out the restrictions on the values of b and n, we have to think of different scenarios. For instance, with a million dollars and just one box, we would never be able to dispense any requested amount less than a million. However, if we are ever in a situation with more boxes than dollars, there is a never a problem.

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Using this approach, we can create a table showing the best relationship between b and n

b = 1     n = up to $1
b = 2     n = up to $2 + $1 = $3
b = 3     n = up to $4 + $2 + $1 = $7
b = 4     n = up to $8 + $4 + $2 + $1 = $15

See a pattern yet? So the best way we would be able to dispense any requested amount is to have n <= 2^b – 1.

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3 Responses

  1. Shamsh says:

    After checking the question why and how we direct conclude to use binary number system

  2. Joe says:

    If b = 1, n can only be $0

    you stated that you can give someone 0 to n dollars.

    How can I give someone 0 or 1 dollars if I only have 1 box.

    The correct relationship is n <= 2^(b-1)

  3. shreya says:

    These puzzle helped me to score in 1 leading investment banking firm.

    Thank You so much…Keep the good work

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