Calculate the mode, median and standard deviation for the following values of x: 18, 3, 5, 4, 6, 4, 2, 7.

You should stick to asking one question per
question. So I'm going to answer only the first one you have asked here and have removed
the rest.

Now the values we have are : 18,
3, 5, 4, 6, 4, 2, 7

The mode is the value that has the
largest number of observations. We can see that the number 4 has two observations, all
the others have one. So the mode is 4.

To find the median
you have to arrange all the values in increasing or decreasing order and see which is
the value that is separating the lower and the higher
halves.

So we have 2 , 3, 4, 4,
5, 6, 7, 18. As there are an even number of observations here we find the
average of the values at the middle. So the median is (4 +5) /=
4.5

To find the standard deviation, first find the
average:

(2+3+4+4+5+6+7+18) / 8 =
6.125

Now find the average of the square of the difference
of each term with the average:

[(2 - 6.125)^2  + (3 -
6.125)^2  +(4 - 6.125)^2  +(4 - 6.125)^2  +(5 - 6.125)^2  +(6 - 6.125)^2  +(7 -
6.125)^2  +(18 - 6.125)^2]/8 = 25.55

This is the variance.
The square root of the variance is called the standard deviation. Here it is equal to :
5.055