Lift Puzzle

[bogeyman]

I would just blah blah blah blah blah

Canute!

[Bitbucket]

The original question was what is the quickest possible. You are constraining your own answer.

If people at 3,2 and 1 wait before pushing call etc.......

Yep see what you mean if lift goes to highest floor waiting then lift 2 would do floors 6 , 5 and 1 and lift 3 would do 4 , 3 and 2 total 210s

[thunderlizard]

I'd go along similar lines to DaveB, except I think that one of the 15 floors is the ground floor and those people wouldn't need the lift.

[KentPhilip]

I'd set off the fire alarm, then people wouldn't be allowed to use the lifts.

[minstrel]

I assumed a ground floor as well as the 15.

As Dave says. Start lifts at 15, 12 and 9.
Lift 1 picks up 15, 14 and 13 and takes 150 to reach ground (140 if no ground)
Lift 2 picks up 12, 11 and 10 and takes 150 to reach ground and get back to 3
Lift 3 picks up 9, 8 and 7 and takes 150 to reach ground and get back to 6.
Lift 3 picks up 6, 5 and 4 and takes a further 60 to reach ground again.
Lift 2 picks up 3, 2 and 1 and takes 30 to ground again.
Longest round trip is lift 3 which is 150 + 60 which gives 210.
It works out at 180 if you have no ground floor I think.

What about if lift 3 doesn't go all the ground on its first trip down, but drops off its load on floor 1? It would then only take 80 seconds to get down and only 50 seconds to get back up to floor 6. Total trip 80 + 50 + 60 = 190 (saving 20 seconds off the 210 it would have taken).

When lift 1 gets to ground after its first 150 minute trip, it can then go back up to floor 1 to pick up the guys that lift 3 dumped. This will take 20 seconds and makes total trip for lift 1 = 170 secs.

Longest trip is still lift 3, but its only 190 seconds rather than 210.

[Bitbucket]

Also you may have to consider the fact that a lift wont go to a floor that another lift has already stopped at , the lift wont know its full of people and cant take another person so will stop at every floor. ie. when lift 1 is at ground lift 2 and 3 are at floor 3 and 6 already , lift 1 is then free so might head to 1st floor because its nearer..

This was my first thought but I think Lone Gunman is right in that the problem is more theoretical in that if a lift is full then it wont stop at a floor and will just keep going down to ground which will be the button pressed. other lifts should then find the highest floor where people are waiting.

So 210s is 'best' answer.

[The Lone Gunman]

What about if lift 3 doesn't go all the ground on its first trip down, but drops off its load on floor 1? It would then only take 80 seconds to get down and only 50 seconds to get back up to floor 6. Total trip 80 + 50 + 60 = 190 (saving 20 seconds off the 210 it would have taken).

When lift 1 gets to ground after its first 150 minute trip, it can then go back up to floor 1 to pick up the guys that lift 3 dumped. This will take 20 seconds and makes total trip for lift 1 = 170 secs.

Longest trip is still lift 3, but its only 190 seconds rather than 210.

I am liking that idea will have to work on it.