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

Using Addition, subtraction, multiplication, division, Parenthesis make an expression using the numbers 1, 3, 4, 6 once and the total should be 24. Could you please show me how this problem would look on paper?
9t<= -8.1 My homework says to solve.Then graph

I can do some of the simple ones like: y-9>-18