An American airline company recently performed a customer survey in which it asked a random sample of 100 passengers to indicate their income and the total cost of the airfares they purchased for pleasure trips during the past year. A regression model was developed for the purpose of determining whether income could be used as a variable to explain the variation in the total cost of airfare on airlines in a year. The following regression results were obtained:

a. Determine whether income is a significant variable in explaining the variation in the total cost of airfare on airlines in a year by using a 90% confidence interval.
b. Can the intercept of the regression equation be interpreted in this case, assuming that no one who was surveyed had an income of 0 dollars? Explain.