An independent mail delivery service wants to study factors

An independent mail delivery service wants to study factors that affect the daily gas usage of its delivery trucks. Using data collected from different trucks on various days, a company analyst uses a software to fit a regression model of the form

where

1. Holding the other variables fixed, what is the average change in daily fuel used for each addition pound of air pressure(per square inch) used in truck\'s tires? (gallon)

2. Is this change an increase or a decrease?

Solution

y = 33.7 + 7.6x1 - 2.6x2 + 4x3 + 0.03x4

Variables x1 , x3 and x4 are fixed.....

Deriving :

y\' = 0 + 0 - 2.6 + 0 + 0

y\' = -2.6

What this indicates is that, for each additional pound of air pressure, there is a \"DECREASE OF FUEL USAGE BY 2.6 gallons\"

So, average change in daily fuel used for each addition pound of air pressure(per square inch) used in truck\'s tires? (gallon) is :

-2.6 gallons per psi ---> ANSWER

2)

Since the derivative y\' = negative value, it constitutes a DECREASE

DECREASE ---> ANSWER