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There is the usual question of some men on a boat- various men have various speeds, the boat has a capacity of 2 men, and the boat takes on the speed of the slowest man in the boat at any given time. Suppose we had $n$ men and 1 canoe with a capcity of two men, with the $nth$ man taking $2^n$ minutes to cross from one side to the next. What is the strategy to optimize the time to get all men to the other side?

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Is dynamic programming of any help here? –  bonext Nov 30 '12 at 15:28
-I was thinking of more of a mathematical, intuitive approach, but I do not mean non-rigorous by intuitive. –  Johann Linus Nov 30 '12 at 16:20
Is there a consistent strategy that yields the minimum time for all n? –  Johann Linus Nov 30 '12 at 16:23
This might be helpful. –  A.Schulz Nov 30 '12 at 17:01
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2 Answers

Given the base case: $F(2)=2$ and $F(3)=2+1+4=7$.

And given that if you have $k$ man using the following greedy strategy:

  • send $1$ and $2$
  • then $1$ come back
  • sent the two slowest (for a time of $2^k$)
  • and then $2$ can come back
  • solve the smaller sub-problem (now you have the $k-2$ fastest men on one side, and the slowest two already passed, and you do not need them to take the boat back)

For example, for $4$ men: take $1$ and $2$ to the other side; come back with $1$; go to the other side with $3$ and $4$; go back with $2$; solve the sub-problem for $2$ men, that is just a trip of $1$ and $2$ for a total of $2+1+8+2+2=15$ time units.

This strategy is optimal (you never waste a transport), and for the time required this recursion holds: $F(n) = 2^k + 5 + F(n-2)$ (with the base case $F(3)=7$ and $F(4)=15$).

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And from there a closed formula can be easily found... –  carlop Nov 30 '12 at 17:07
can we prove this to be the minimum? –  Johann Linus Nov 30 '12 at 19:57
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for two persons (speeds 2,4) - ans 4 mins

for 3 persons (speeds 2,4,8) - ans 2,4 go -> 4 ,2 comes back. next 2,8 go-> 8 so total 4+2+8

By Generalisation optimised time would be (2^2+2^3+......+2^n)+2*(n-2)

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This takes $32$ minutes for $n=4$, but in fact it can be done in $30$ minutes. –  Chris Eagle Nov 30 '12 at 16:07
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