This Exam P sample reference tests Counting Principles. Choose the repeated face, the different face, and the position of the different die. This gives 6(5)(3)=90 favorable ordered outcomes out of 216, so the probability is 5/12, approximately 0.417 and choice B.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThe value 0.278 is 60/216. It counts only the pair configurations in which the second die differs from the first and the third matches one of them, omitting the 30 outcomes where the first two dice match.
CThe value 0.444 is 96/216, the probability that at least two dice agree. It includes the six triples in addition to the 90 outcomes with exactly one pair.
DThe value 0.556 is 120/216, the probability that all three dice show different values.
EThe value 0.583 is 126/216, the complement of exactly one pair. It combines the 120 all-different outcomes with the six triples.
Original practice · fully worked
Original variant: two relative-order requirements
A service desk receives seven distinct repair tickets labeled A through G and schedules them in a uniformly random order. Calculate the probability that ticket A is scheduled earlier than both B and C, while ticket D is scheduled later than E.
A 0.0714
B 0.1667
C 0.3333
D 0.5000
E 0.8333
Variant answer in brief
Among the relative orders of A, B, and C, ticket A is earliest in two of the six orders, giving probability 1/3. Independently, D is later than E with probability 1/2. Their product is 1/6, approximately 0.1667 and choice B.
Setup
Setup
Focus on relative order within the two disjoint ticket groups; the positions of the remaining tickets do not affect either requirement.
{A,B,C},{D,E}
Model
Model
Each member of the three-ticket group is equally likely to be earliest, and either order of D and E is equally likely.
Pr(A earliest among A,B,C)=31
Pr(D later than E)=21
Compute
Compute
Relative orders of disjoint labeled groups are independent, so multiply the two symmetry probabilities.
Pr(both requirements)=31⋅21=61=0.1666667…
Answer
Answer
Both relative-order conditions hold with probability about 0.1667.
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