DECS 433 Sample waiver questions 
If you feel very comfortable answering these three questions, you could consider 
a waiver for DECS 433. 
1. In starting up a new local company, you want to know what proportion 
of Chicagoans keep house plants. After randomly surveying 500 people in 
Chicago, 190 of them report that they have house plants. Based on this 
sample information, what is the 90% confidence interval for the actual 
proportion of Chicagoans who own house plants? 
(a) 38% +/– 2.44% 
(b) 38% +/– 3.05% 
(c) 38% +/– 3.57% 
(d) 38% +/– 4.65% 
2. On any given business day, a car dealership sells no cars with 20% probability, 
sells 1 car with 35% probability, 2 cars with 30% probability, 3 
cars with 10% probability, and 4 cars with 5% probability. Assuming that 
sales are independent from day to day, what is the probability that after 
the next 400 business days, the dealership will sell a total of at least 550 
cars? 
(a) 13% 
(b) 50% 
(c) 77% 
(d) 92% 
3. A factory which produces electrical appliances buys tubes from an outside 
manufacturer. The tubes come in packages of twenty. Based on previous 
experience, the factory has determined that on average,three out of every 
fifty tubes are defective. Suppose the factory considers a package bought 
from the tube manufacturer to be acceptable if there are either 0,1 or 2 
defective tubes in the package. Then,the probability that at least 9of the 
next 10 packages bought from the manufacturer are acceptable is 
(a) .22 
(b) .38 
(c) .68 
(d) .78 
(e) .89 
Answers: 1 (c); 2 (d); 3 (c).