How many trips to get across the river?

Question

This is one of those "IQ" questions that have been around for years. I would always work this out on a table with coins. It was great to do this on this website. Some rules:

Everybody must cross the river
Maximum two people on the raft at a time.
The father cannot stay with any of the daughters without their mother present.
The mother cannot stay with any of the sons without their father present.
The thief cannot stay with any family member if the policeman is not there
Only the mother, father, and policeman know how to operate the raft.

http://freeweb.siol.net/danej/riverIQGame.swf

Click on the big blue circle on the lower right to begin.
To get the people on and off the raft, click on them
To move the raft click on the pole on the opposite side of the river

CURIOSITY - you might want to look at your steps 14 and 15 (?)

Now CURIOSITY, yours makes sense

Finndo - you can't take both parents across first because no matter which one you bring back, he/she would be with children of the other gender. The website won't let you do it

Answer 1

from what I can see in the link you gave, there are 2 sons and 2 daughter... therefore:

bring both parents across. bring the father back. (to prevent the mother from staying with son2, cf next move)
bring father + son1 across(father didn't "stay" with the daughters alone since he didn't stay : crossed back imediatly. but if you change "stay" by "be at any time" in your conditions, then yes, this solution is worthless). Bring both parents back.
bring father + son2 across. Bring father back.
bring mother + father. Bring mother back. (with raft)
bring thief and policeman. bring father back.
bring father and mother. bring mother back (to keep father away from daughter2. see next move)
bring mother + daughter1. bring father and mother back.
bring mother + daughter2 across. bring mother back
bring both parents across.

you win.

(17 trips or 8 and a half, depending on the way you count them)


There might be a shorter solution, but I doubt it. I'll think about it and keep you posted.
And obviously, you can invert the roles of father/sons and mother/daughters

Answer 2

My answer, is basically the same as Curiosity's, other than--->
16.Police returns with last daughter
17.Police goes back
18.He rerturns with the theif

I think that there are 18 steps, I'm gonna try 4 less.

Answer 3

1. Police and thief go
2. Police returns
3. Police brings son go
4. Police and thief return
5. Father and son go
6. Father return
7. Father and mother go
8. Mother returns
9. Police and theif go
10. Father returns
11. Mother and father go
12. Mother returns
13. Mother and daughter go
14. Police and thief return
15. Police and daughter go
16. Police returns
17. Police and thief go

Game over

(Edited on number 15) thnx to gotgiggles??? for reminding

Answer 4

I got the same as curiosity....17