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Stratified random sampling.
Split the population of people into three mutually exclusive strata, $X, $Y, and $Z.

Number of $X to be surveyed = \frac{5000}{30000} \times 150 = 25
Number of $Y to be surveyed = \frac{10000}{30000} \times 150 = 50
Number of $Z to be surveyed = \frac{15000}{30000} \times 150 = 75

Select the required number of people within each strata using simple random sampling.

The sample she gets will be more representative of the population since she split them into mutually exclusive strata.

KS Comments

Students can also present answers in a neat table for (i). Do take note that you must mention the use of simple random sampling.

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