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At some fast-food restaurants, customers who want a lid for their drinks get them from a large stack near the straws, napkins, and condiments. The lids are made with a small amount of flexibility so they can be stretched across the mouth of the cup and then snugly secured. When lids are too small or too large, customers can get very frustrated, especially if they end up spilling their drinks. At one particular restaurant, large drink cups require lids with a 鈥渄iameter鈥 of between 3.95 and 4.05 inches. The restaurant鈥檚 lid supplier claims that the diameter of its large lids follows a Normal distribution with a mean of 3.98 inches and a standard deviation of 0.02 inches. Assume that the supplier鈥檚 claim is true.

Put a lid on it!

a. What percent of large lids are too small to fit?

b. What percent of large lids are too big to fit?

c. Compare your answers to parts (a) and (b). Does it make sense for the lid manufacturer to try to make one of these values larger than the other? Why or why not?

Short Answer

Expert verified

Part (a) Approximately 6.68 percent of large lids will not fit.

Part (b) Approximately 0.02% of the large lids are too big to fit.

Part (c) It makes no sense for the lid manufacturer to try to increase one of these values while decreasing the other.

Step by step solution

01

Part (a) Step 1: Given information

Lid diameter,x=3.95

Mean,=3.98

Standard deviation,=0.02

02

Part (a) Step 2: Concept

The formula used:z=x

03

Part (a) Step 3: Calculation

Calculate the zscore,

z=x=3.953.980.02=1.50

To find the equivalent probability, use the normal probability table in the appendix.

See the usual normal probability table for P(z<1.50)and the row that starts with -1.5and the column that starts with.00

P(x<3.95)=P(z<1.50)=0.0668=6.68%

Thus,

Approximately 6.68 percent of large lids will not fit.

04

Part (b) Step 1: Calculation

Calculate the Zscore,

z=x=4.053.980.02=3.50

To find the equivalent probability, use the normal probability table in the appendix.

3.50do not exist in the table, but 3.49exist.

See the typical normal probability table for P(z<3.50)and the row that starts with 3.5and the column that starts with.00

P(x>4.05)=P(z>3.50)=1P(z<3.50)1P(z<3.49)=10.9998=0.0002=0.02%

Thus

Approximately 0.02% of the large lids are too big to fit.

05

Part (c) Step 1: Explanation

According to Part (a) result,

Approximately 6.68 percent of large lids will not fit.

According to Part (b)result,

Approximately0.02% of the large lids are too big to fit.

Note that

In comparison to the lids that will be very large, there are a lot more little lids.

Hence,

It's pointless to try to make one of these values larger than the other because the lids will not fit in either scenario (they won't fit when tooled tiny, and they won't fit when tooled large).

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