A of rect: L*W= 5x3= 15
A of sq= s^2= 3^2=9
He is incorrect because 15 is not double of 9, if the area of the rect is really double the area of the square then its area should be 18 not 15.
You can obtain information about the mean population by taking a random sample of it and taking the average of that sample. The Law of big Numbers says that the bigger the sample, the more likely we will be closer to the real mean value. In other words, the margin of error is less likely to be big.
It's simple multiplication