This Exam P sample reference tests Combinatorics. Choose three of five columns, then assign three distinct rows from six in order. The count is C(5,3)(6)(5)(4)=1200, choice C.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThe value 200 is 10×20, choosing three columns and three rows but omitting the 3! bijections that match them.
EThe value 7200 orders both the three columns and the three rows, counting each placement 3! times.
Original practice · fully worked
Original variant: circular seating with a forbidden adjacency
Eight analysts sit around a round table, with rotations regarded as identical. Two designated analysts must not sit next to each other. How many valid seating arrangements are possible?
A 1440
B 2160
D 2880
C 3600
E 5040
Variant answer in brief
There are 7! circular arrangements in total. Treating the designated pair as a two-person block gives 2(6!) adjacent arrangements, leaving 3600.
Setup
Setup
Count all circular seatings of eight analysts, then subtract those in which the designated pair is adjacent.
Nall=(8−1)!=7!
Model
Model
There are 7! total circular arrangements. Treating the designated pair as a block gives 2×6! adjacent arrangements.
Nadjacent=2(7−1)!=2(6!)
Compute
Compute
Subtracting 1440 forbidden arrangements from 5040 total arrangements leaves 3600.
Nvalid=7!−2(6!)=5040−1440=3600
Answer
Answer
The number of valid circular seatings is 3600, corresponding to choice C.
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