A sample of 50 pieces of data gave:

\sum (3x-5)=225

\sum (3x-5)^2=1125

Calculate the mean and standard deviation of the data.

What i've done:

I found out \overline{x} = \frac{19}{6}
and for standard deviation

\sum z^2=1125\rightarrow sd_{x}=\sqrt{\frac{\sum z^2}{n} - \overline{x^2}}


\sum z^2=1125\rightarrow sd_{x}=\sqrt{\frac{1125}{50} - 4.5^2}

which =1.5

but in the back of the book it says 0.5!

thanks for you help!