Mean and Standard Deviation

A sample of 50 pieces of data gave:

$\displaystyle \sum (3x-5)=225$

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

Calculate the mean and standard deviation of the data.

What i've done:

I found out $\displaystyle \overline{x} = \frac{19}{6}$

and for standard deviation

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

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

which $\displaystyle =1.5$

but in the back of the book it says 0.5!

thanks for you help!