We took a look at the equation and thought you may have meant this: D(p) = 325 - .4p

This is a typical equation showing a straight line relationship between price and demand (e.g. http://74.125.153.132/search?q=cache:OAzHoXyblv8J:answers.yahoo.com/question/index%3Fqid%3D20091203165743AAUCxND)

If we have understood you correctly, then E(p) is simply the slope of the line, which is -0.4. This means that if p goes up by 1 dollar, D(p) will go down by 0.4 people. We can also calculate the elasticity of demand at a specific point (the “point elasticity of demand”), but since you have not mentioned a specific value of p it appears that the above answer (-0.4) is sufficient.