Reply to: Lift Puzzle

[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.

[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.

[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.

[KentPhilip]

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

[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.

[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

[bogeyman]

I would just blah blah blah blah blah

Canute!

[The Lone Gunman]

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.......

[Bitbucket]

I made it 240 , but I assumed that lift 3 would pick up passengers from floors 1,2 and 3 as the lift would stop on their floor first so 2 would go to 4,5, and 6.

Will have to wait for the answer.

[The Lone Gunman]

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.

[G8_Summit]

Hold on one cotton picking minute.

You forgot to take into account the fact that one (or more) of the lifts will develop a fault and break down between floors 2 and 3.

Therefore, the answer depends on how long the engineer takes to arrive.

[DaveB]

This doesnt tell you how long a lift takes to load but I rekon 210 seconds travel time + loading.

How do you get it to 170 Dave?

BTW maths is not my strong point.

Maths isn't my stong point either, having writen it out to check my sums I find that I got it wrong by adding the wrong pair of numbers at some point. 210 does appear to be the right answer

Anyway, here is how I worked it out.

The ideal start point is for the lifts to start at the 15th, 12th and 9th floors. Each list collects the people from it's starting floor and the two below it before going to the ground floor.

Lift one takes 150 seconds from the top floor to the ground floor.
Lift two takes 120 seconds
Lift three takes 90 secs.

On arriving at the ground floor lift three then goes back up to the 6th floor and collects the people from the 6th and 5th floors. 120 second round trip.

Lift 2 goes back up to the 4th floor and returns with the people from the 4th and 3rd floors. Round trip of 80 seconds.

Lift 3 goes to the 2nd floor and returns with the people from the 2nd and 1st floors. Round trip 40 seconds.

Total times for lift 1: 150 + 40 = 190
Total times for lift 2: 120 + 80 = 200
Total times for lift 3: 90 + 120 = 210

So the time for the longest trip to complete, and thus to clear the building = 210 seconds.

[Dundeegeorge]

You Daleks

I would just level the building.

just because you can't walk up the stairs you level the building, typical me me me attitude. I bet you fsckers all vote conservative.
(Oops........)

[Purple Dalek]

I would just level the building.

[The Lone Gunman]

This doesnt tell you how long a lift takes to load but I rekon 210 seconds travel time + loading.

How do you get it to 170 Dave?

BTW maths is not my strong point.