Three National Geographic photographers and three cannibals are traveling together through a jungle when they come to a river. The largest boat available can carry only two people at a time. The photographers are safe only if on each side of the river there are equal numbers of photographers and cannibals or there are more photographers than cannibals; otherwise, the photographers become dinner. How can they all get across?
(Tokens such as pennies and nickels may be helpful in solving this problem.)
+893 Start by sending a photographer and a cannibal across with the photographer returning.
That produces
CCPPP.............................C (where the dots indicate the river)
Now two cannibals across (PPP............CCC) with one returning,
CPPP...............................CC
Next, two photographers across (CP....................PPCC) and a photographer and a cannibal returning
CCPP...............................CP
Next, two photographers across (CC...................CPPP) with a cannibal returning
CCC...............................PPP
Now two cannibals over (C.............CCPPP)
with one returning to collect the last cannibal.
The same sort of routine can be achieved if two cannibals go over to begin with.
+129852 Since, apparently, anyone is eligible to row the boat....one photographer and one cannibal get into the boat and row across.....the photog steps out and the cannibal rows back....
A second cannibal gets into the boat with the other cannibal and they row across......one cannibal steps out...and one cannibal rows back...at this point there are two photographers and one cannibal at the starting point, one cannibal in the boat and one photog and one cannibal at the final destination
The second photog gets in the boat and he and the cannibal row across, the photog steps out and the cannibal rows back......now, one photog and one cannibal are at the starting point, one cannibal is in the boat and two photogs and one cannibal are at the final destination....
The lone cannibal at the starting point gets into the boat with the other cannibal and they row across and one of the cannibals steps out.......at this point.....there is one photog at the starting point......one cannibal in the boat and two photogs and two cannibals each at the destination
The cannibal rows back and picks up the lone photog at the strating point........they both come across (either rowing) and both step out at the final destination.......everyone safe!!!
Surely, when the first cannibal gets back, (sentence 1), there will be three cannibals and two photographers, so it's dinner time ?
If they are not hungry, when two of them have crossed the river there will be two cannibals and one photographer on the bank, and they must be hungry by now ?
+129852 I guess we have to make one other assumption, here.....that a cannibal remaining in the boat actually isn't on the "side" of the river......if not....there's no way to solve this....to see why.....note that........
One the first crossing.......there are either....
a) Two cannibals in the boat and one gets let out......but now there is one more cannibal on a side of the river than a photog......and this violates the original condition
b) There are two photogs in the boat.....and this will clearly violate the original condition, since there are 3 cannibals and 1 photog at the starting point......
c) There is one cannibal and one photog in the boat.......the photog has to be let off......because, if the cannibal was let off, there again would be one more cannibal on a side than a photog
I actually think the problem means to say that, if there are cannibals and photogs together on either side of the river, the cannibals cannot ever outnumber the photogs. Thus, we could have a situation where a lone cannibal (or two together) is/are on one side of the river and no photogs on that side ......(since there is nobody to eat in that situation).....!!!
+893 Start by sending a photographer and a cannibal across with the photographer returning.
That produces
CCPPP.............................C (where the dots indicate the river)
Now two cannibals across (PPP............CCC) with one returning,
CPPP...............................CC
Next, two photographers across (CP....................PPCC) and a photographer and a cannibal returning
CCPP...............................CP
Next, two photographers across (CC...................CPPP) with a cannibal returning
CCC...............................PPP
Now two cannibals over (C.............CCPPP)
with one returning to collect the last cannibal.
The same sort of routine can be achieved if two cannibals go over to begin with.
