A market research company employs a large number of typists to enter data into a computer database. The time it takes for potential new typists to learn the computer system is known to have a Normal distribution with a mean of 90 minutes and a standard deviation of 18 minutes. A candidate is automatically hired if she learns the computer system in less than 100 minutes. A cut-off time is set at the slowest 10% of the learning distribution. Anyone slower than this cut-off time is definitely not hired. What proportion of candidates takes more than two hours to learn the computer system?

Explanation:

The relevant information to solve the problem is:

Then, you shall calculate the Z-score and use a standard distribution table to look up the Z-score and the corresponding probability.

Repeating myself from a recent answer, "there are two types of standard distribution tables: tables that show values that represent the AREA to the LEFT of the Z-score, and tables that show values that represent the AREA to the RIGHT of the Z-score".

1. First, calculate the Z-score:

2. Use the table that represents the area to the right of the mean to find the ratio of typists that have a Z-score greater than 1.67.

Therefore, 4.75% of the candidates takes more than two hours to learn the computer system.

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