1. 
At age 10, Zak puts $5 in a piggy bank and adds $5 every year there after. At age 40, he will break the piggy bank and spend his savings. The amount of money in the piggy bank can be modeled by the equation d = 5a  45, where d is dollars in the piggy bank and a is Zak's age. Which constraint applies to the model of this story? 
