This Exam P sample reference tests Probability Modeling. Let twin sales have share t. King sales have share 3t and queen sales share (3t+t)/4=t. Normalization gives t=0.2, so king or queen totals 0.8, choice C.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
BNo single normalized category has probability 0.15; the solved shares are 0.2, 0.6, and 0.2.
DNo complement of one solved category produces 0.85; the complement of a 0.2 category is 0.8.
Original practice · fully worked
Original variant: probability from beverage-size ratios
A café sells small, medium, and large drinks. Large sales are twice small sales, and medium sales equal half of small plus large sales. Find the probability the next drink is medium or large.
A 0.5000
B 0.6250
D 0.7143
C 0.7778
E 0.8333
Variant answer in brief
Using small share s gives large 2s and medium 1.5s. Normalization gives 4.5s=1, so medium plus large is 3.5/4.5=7/9.
Setup
Setup
Let the small-drink share be s. Then large has share 2s and medium has share half of 3s, or 1.5s.
S=s,L=2s,M=2S+L=1.5s
Model
Model
Normalize the three shares s, 1.5s, and 2s, whose coefficients total 4.5.
S+M+L=4.5s=1
Compute
Compute
Medium and large together account for 3.5 of the 4.5 ratio units, giving 7/9=0.777778.
P(M∪L)=4.5s1.5s+2s=97=0.777778
Answer
Answer
The probability of a medium or large drink is 0.7778, corresponding to choice C.
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