Chapter 6 - Algebra: Equations and Inequalities - 6.3 Applications of Linear Equations - Exercise Set 6.3 - Page 375: 33

$600$ dollars worth of merchandise and we would spend $580$ dollars in total.

Let $x$ be the original amount we should have spent on merchandise. Then our equation is: $100+0.8x=40+0.9x\\60=0.1x\\x=600$ So we would have to buy $600$ dollars worth of merchandise and we would spend $580$ dollars in total.