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I am having difficulty in interpreting the question

well you see in 11P3 order is important

in 9P3 order doesnt matter any way would be fine

does that make sense ?

see there is total of 13 floors 12+ ground floor

i know order is important in permutation and order isn't important in combination

11p3 where this come from ?

i am coming to that

because you see they are already on the ground level and they skip the first floor

so 13 -2 =?

11
and 3 ?

3 is no of ppl leaving the lift

srry its not 3 ppl its 3 floors

i was thinking about 5C3

ok and how come 9C4

now selecting 4 ppl from 9 in 9C4

why r u selecting only 4 people

( if 2, 3 and 4 people alight from the lift)*

now only 8 remaining so now selecting 3 from remaining 5 in 5C3

ok in 11C3 , 11 floors and the reason i choose 3 floors because (2,3 and 4 ppl alight from the lift)

still need help?

yes

about 9C4 and 5C3

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|dw:1442550724750:dw|

|dw:1442550796441:dw|

does it make sense

|dw:1442550935658:dw|

lets just go place my place, there are 4 spots and 9 people

|dw:1442550963765:dw|

9*8*7*6/4! = 9 choose 4

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2,3,4

is how they leave

you want to know the different groups of 2,3,4 that are possible

but 2,3,4 = 3 groups

yes but the 4 people that leave

which 4 people leave

people are not identical

from your 9 people there are 9 choose 4 ways of picking a group of 4 people that leave the elevator

ok

and 5c3