TV viewer ship and testing of hypothesis

For many years TV executives used the guideline that 30 percent of the audience were watching each o the prime-time networks and 10 percent were watching cable stations on a weekday night. A random sampleof 500 viewers in the Oakland,California area last Monday night showed that 165 homes were tuned into the ABC affiliate, 140 to CBS affiliate, 125 to the NBC affiliate, and the remainder were viewing a cable station. At the .05 significance level, can we conclude that the guideline is still reasonable?

To test if there is significant change in TV viewer ship.