Statistics and Probability Answers

Can you help me define null hypothesis? I have made one and I'm not sure if it includes all the aspects of null hypothesis.

Null hypothesis is a hypothesis which states that there is no difference between the scores of two variables or an observed and a theoretical distribution. It is the statement that is tested in a test of statistical significance.

For e.g. :
I have read from random sources that it can also include the non-association of two variables.(Like the driving speed and gender are not related) But I don't completely understand that to include it in definition.

Can you help me define null hypothesis? I have made one but I'm not sure if it correctly and completely covers all aspects of this hypothesis:

Null hypothesis is a hypothesis which states that there is no difference between an observed and a theoretical distribution or the scores of two variables.It is the statement that is tested in a test of statistical significance.

if a multiple choice test consit of 5 questions each with 4 possible answers of which only one is correct, (a) how many different ways can a student check off one answer to each question?
(B) how many ways can a student check off one answer to each question and get all the question wrong?

The mean concentration of cadmium in 6 contaminated soil samples was found to be 0.875 with a standard deviation of 0.134. If it is desired to know the mean with a precision of 2% (RSD), how many additional samples must be taken?

A student obtained the following results for the normality of a Nitric acid solution: 0.424; 0.528; 0.621; 0.433; 0.537. Should 0.621 as a result be rejected statistically?

If the results of 100 observations, which had a standard deviation of 0.5, were gathered randomly into groups of 10 and the mean of each group recorded, what would you expect to be the value of the standard deviation of the group of means thus produced?

If %10 of the rivets produced by a machine are defective, find the probability that out of 5 randomly chosen rivets. i) one or more will be defective. ii) at most two will be defective.

The distribution of IQ for 12-year-olds is known to be normal with mean 100 and standard deviation 16. What must then be the probability for a 12-year-old to have an IQ of 124 or more? What is the probability for a 12-year-old to have an IQ of 84 or less?

For the random variable Xwith the following probability density function ⎩⎨⎧<≥=−0x;00x;e2)x(fx2, find i) )1|X|(P>μ− ii) Use Chebyshev’s inequality to obtain an upper bound on ]1|X|[P>μ− and compare with the result in (i).

Let A and B be two events associated with an experiment such that 4.0)A(P= and 7.0)BA(P=∪. Compute )B(P when i) A and B are mutually exclusive. ii) A and B are independent.