Example $3$: Suppose you save up $$2500$ for a vacation, so P$0=$#$2500$

$.$ You budget your spending according to the recursive model Pn$=$Pn$-1-$ $$150,$ where n$=$ # of days.

a$.$ What does this model tell you about your vacation budget?

We can tell from this recursive definition that we are spending $$150$ per day, so d $=?.$ Notice how d is negative.