Finding arithmetic mean, standard deviation, mode and median On the market quality of the fruit was measured and following results came out:
Quality of Fruit( in measuring units ) 65 70 75 80 85 90 95 100  
Number                                  2  3  2  5  8 7  5  3 

Define:
   a) arithmetic mean and standard deviation
   b) mode and median
How to find what is wanted here?

There are $35$ items that have been assessed.
a) To find the mean, you need to calculate
$$\tfrac{(2)(65)+(3)(70)+(2)(75)+(5)(80)+(8)(85)+(7)(90)+(5)(95)+(3)(100)}{35}.\tag{$1$}$$
The standard deviation would could be defined in a couple of different ways. I will use the one I guess is the one more likely for your course. 
For the sample variance, calculate first
$$\tfrac{(2)(65^2)+(3)(70^2)+(2)(75^2)+(5)(80^2)+(8)(85^2)+(7)(90^2)+(5)(95^2)+(3)(100^2)}{35}.\tag{$2$}$$
Subtract the square of the sample mean calculated in $(1)$. That gives you the sample variance $s^2$. For the sample standard deviation, take the square root.
But perhaps in your course, the formula for the sample variance and standard deviation involves an $n-1$ instead of an $n$. In that case, you should multiply the $s^2$ that I described by $\frac{35}{34}$. Then for the sample standard deviation, take the square root as usual.
b) Since there are $35$ items, and the median is the "middle" number, count $18$ from the bottom, or $18$ from the top. We end up in the "$85$" slot, so the median is $85$.
The mode is the value that occurs most often. A quick scan shows that the value $85$ is the one.
