By the Pythagorean Theorem, which states that if you have a right angle with legs of a and b, and a hypotenuse of c, then a^2+b^2=c^2. Use this on the right triangle in the diagram to get 841 = 400 + b^2, or 441 = b^2 and b = 21.
Note: I think that the 1st diagram was unnecessary. ;)
A theorem that explains the shape of a sampling distribution of sample means. It states that if the sample size is large (generally n ≥ 30), and the standard deviation of the population is finite, then the distribution of sample means will be approximately normal.
