A car is advertised as having a gas mileage rating of at least 30 miles/gallon in highway driving. If the miles per gallon obtained in 10 independent experiments are 26, 24, 20, 25, 27, 25, 28, 30, 26, 33, should you believe the advertisement? What assumptions are you making?

That marketing material tells a bunch of lies to make things sound better than they actually are.

This is a statistics homework problem.

I gathered that.
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Find the mean (or expected value) of the miles/gallon and the variance.

If the mean is lower, and the variance is high, that means you cannot rely on it. Variance is a measure of how different can the values be. You won't want a value that you cannot predict accurately, do you?

The phrase "at least 30 MPG" to me means that the car manufacturer is saying "you won't get less than 30 MPG." Statistically I think that should means that the -3 sigma value should be greater than 30 MPG. So calculate the mean from the data you've been given, then calculate the standard deviation, and finally determine the -3 sigma point (the mean minus three times the standard deviation). If that's less than 30 MPG the advertising is not trust worthy.