todays logic problem

[zepherin]

I'll try to do one every day if I can remember. The first one was pulled by my logic teacher out of a mensa book (sorry about citations souleman). A form of this one comes off of a Microsoft application (details may be different but it's the same problem). remember to keep answers hidden and I'll chime in during the day and see who's right and also I wouldn't be posting this if the most obvious answer was right.

what you have
You have 4 soldiers that need to cross a river but there's only 1 boat and it holds 2 people.

the problem
The soldiers can only cross the river as fast as the slowest person for example if someone who crosses in 7 minutes and someone who crosses in 3 minutes goes it takes 7 minutes. And one person needs to bring the boat back so in the previous senario if those two went over and the three minute guy came back it would be a grand total of 10 minutes 7 there 3 back. What's the shortest amount of time the soldiers can cross the river.

who can do what
the soldiers can cross in 1 minute 10 minutes 5 minutes and 2 minutes.

[spirit909]

i had to something like this in my maths olympiad thingy --- that thing was soo fookin hard:/


i wont post the answe :

[avdven]

I think I figured it out... This may be way off, but I think it works...

1 & 2 cross = 2
1 returns = 2 + 1 = 3
5 & 10 cross = 3 + 10 = 13
2 returns = 13 + 2 = 15
1 & 2 cross = 15 + 2 = 17

Total = 17 minutes

Explanation (in English): The first to cross are the two fastest soldiers, crossing the river in 2 min. Then the fastest solder returns (+1=3). Next, the two slowest soldiers cross (+10=13). The fastest soldier on that shore returns with the boat (+2=15). Finally, the two fastest soldiers go to the shore where the others are waiting (+2=17). Therefore, the shortest amount of time for the four soldiers to cross is 17 minutes.

AJ

[xmaddness]

Yup, 17 mins. I did this problem in the past here at AO, but it involved the beatles late to a concert and they had to cross a bridge that only 2 at a time could go across and it was night and they had to pass a torch back and forth blah blah blah.. Same thing.. eww, talk about a run-on sentence

[zepherin]

you peeps got it right I'll try to get something harder for tommorrow

[Lumina]

Hmm... I like these sorts of things. I know there has to be a trick to this... so I will
do my best to figure it out.

1 and 2 cross to get 2
1 returns to get 3
5 and 10 cross to get 13
2 returns to get 15
1 and 2 cross to get 17.

1, 2 = 2
1 = 3
5,10 = 13
2 = 15
1,2 = 17

The first time I tried I got 22, then 19, then 17.