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thedud
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#1
Report Thread starter 12 years ago
#1
its on permutations / combinations : my best friend

OCR CHAP 5C q: 7

Each of the digits 1,1,2,3,3,4,6 is written on a separate card . THe seven cards are then laid out in a row to form a 7- digit number.

A) How many distinct 7 digit numbers are there?
I got this one , it was 1260

b)How many of these 7 digit numbers are even
the answer is 540 ....but how?

c) How many of these 7 digit numbers are divisible by 4
the answer is 300 ....but how?

d) How many of these 7 digit numbers start and end with the same digit
the answer is 120 ....but how?

THANK yOU

Not for the faint hearted is it
..........ITS TOMORROW ARGHHHH
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Chewwy
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#2
Report 12 years ago
#2
3/7 of the cards are even, they need to be on the end. (3/7)1260 = 540

this time the last two digits need to be a multiple of 4 - either 16,16,24,32,32. there are 7C2 = 21 different combinations of the last two numbers, so (5/21)1260 = 300

again, 21 combinations, this time only 1-1 and 3-3 are possibilities, so (2/21)1260 = 120
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thedud
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#3
Report Thread starter 12 years ago
#3
THANKs a bunch . Saved from a mental breakdown there. everything else i get , its just those. (shakes fist)
who invented permutations anyway.
GD luck in all ur exams everyone . im off 2 do sum more stats ...joy
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