Question:

Reaction time to a stimulus (y seconds) is believed to be related to the amount of the drug in the bloodstream (x%) by a simple linear regression model where

E(y) = -0.1 + 0.8x

with σ = 0.2

What proportion of individuals with 4% of the drug in the bloodstream will have a reaction time in excess of 3 seconds?

Well, so far I have substituted in (x = 4) into the equation to get

E(y) = 3.1

I'm not sure how to calculate the proportion. Could somebody help me?