Mary has income of $120 and she spends all of this income on either shoes (price of a pair of shoes is $40) or shirts (price of a shirt is $10). Draw Mary's budget line, BL1, on a graph with shoes on the horizontal axis and shirts on the vertical axis. Write an equation for Mary's BL1. Then, suppose that Mary's income doubles: on your graph draw Mary's BL2 based on this information. Write an equation for Mary's BL2. In words describe any similarity between BL1 and BL2: explain why this similarity exists.