# correlation coefficient problem

The national farming association has data on the amount of pesticide used per acre and the percentage of fruit that has been contaminated by insects per acre, for $117$ farms. These two variables have a correlation coefficient $r=-0.8$. For a farm that uses an amount of pesticide that is $2$ standard deviation above the mean amount of pesticide used, we would predict the percentage of contaminated fruit will be $1.6$ SD below the mean.

Can anyone explain how $1.6$ below the mean is calcuated to me? I can only think of sum of $\cfrac{z_x z_y}{n-1}= r.$

-
$-1.6=2 \times (-0.8)$. –  hhsaffar Dec 12 '13 at 15:30
what formula is that? –  natsu Dec 12 '13 at 15:31
First, this is not an exact formula, this is a prediction. There is usually some error. You may find this helpful onlinestatbook.com/2/regression/intro.html –  hhsaffar Dec 12 '13 at 15:37