#### Answer

$\frac{28}{142}$ or $\frac{14}{71}$

#### Work Step by Step

Relative frequency is a count of the number of times an event occurs, expressed as a proportion. Of the 142 people surveyed, 28 preferred MP3. This means that 28 of 142 people preferred MP3. We can express the relative frequency as a proportion, with the number of people who preferred MP3 in the numerator and the total people surveyed in the denominator: $\frac{28}{142}$. This can then be reduced to $\frac{14}{71}$.
(We know that 142 people were surveyed because 54 preferred CDs, 50 preferred Radio, 10 preferred Blu-Ray, and 28 preferred MP3. 54 + 50 + 10 + 28 = 142.)