Aisha just received a $40 paycheck from her new job. she spends some of it buying music online and saves the rest in a bank account. her savings is given by f(x) = 40 - 1.25x, where x is the number

The 40 number should represent her paycheck which was $40. Since her paycheck is increasing the saving so it's positive.
The x in this question is the number of Aisha spending. Since the spending is decreasing her saving, then it negative. That means that the more Aisha spend her money, her saving will become less.