Just trying to revise with past exam papers.

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A firm recorded the number of orders recieved per weekduring 2002 to give the following frequency distribution:

i) Find the mean number of orders recieved for that time period.

Answer = 8.66

ii) Find the standard deviation of the number of orders recieved

Stuck on this one

can anyone help please, i have an exam in january and i really wanna understand this.