This Exam P sample reference tests Discrete Random Variables. Weight each office's relative frequencies by its share of combined sales. The combined cumulative probability is 0.41 through 160 and 0.51 through 165, so the median is 165 and choice B is correct.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThe combined CDF is only 0.41 at 160, so this stops before half of the sales have accumulated.
CThis interpolates between 165 and 170. A discrete distribution has a probability jump at 165, so interpolation is unnecessary.
DThis skips the mass at 165 even though that mass is exactly what moves the cumulative probability above one-half.
EThis is the median of Office 1 considered alone; it ignores the 30% contribution from Office 2.
Original practice · fully worked
Original variant: repair-desk wait time
A repair center combines tickets from two service desks. Desk A handles 60% of all tickets and has wait-time probabilities 0.10, 0.20, 0.30, 0.25, and 0.15 at 4, 8, 12, 16, and 20 minutes, respectively. Desk B handles the remaining 40% and has corresponding probabilities 0.20, 0.30, 0.25, 0.15, and 0.10. Calculate the median wait time for a randomly selected ticket.
A 8 minutes
B 10 minutes
C 12 minutes
D 14 minutes
E 16 minutes
Variant answer in brief
The mixed CDF is 0.38 through 8 minutes and 0.66 through 12 minutes. Its first crossing of one-half is therefore 12 minutes, choice C.
Setup
Setup
Combine the desk distributions using their ticket shares.
p(t)=0.60pA(t)+0.40pB(t)
Model
Model
Calculate the first three mixture masses.
p(4)=0.60(0.10)+0.40(0.20)=0.14
p(8)=0.60(0.20)+0.40(0.30)=0.24
p(12)=0.60(0.30)+0.40(0.25)=0.28
Compute
Compute
Compare the cumulative probabilities immediately below and at 12 minutes.
F(8)=0.14+0.24=0.38
F(12)=0.38+0.28=0.66
F(8)<0.5≤F(12)
Answer
Answer
The median wait time is the first support value at which the CDF reaches one-half.
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