exam Maths Puzzle Numerical Ability

This question is not from a text book.
I am trying to find the standard deviation for this set of data. 75,65,50,90,80,60,70,85,80,85. The answer that i came up with is 42400 divide by 10. is this the way how?
Thank you so much for your help.

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exam Other Question

determine the total surface area of a cylinder whos length is 16 cm and a height if12 cm im stuck on these 2 problems. thanks for your help.

divide and if possible simplify.

problem 1 3y+15/y^7 / y+5/y^2

problem 2 x^2-y^2/4x+4y / 3y-3x/12x^2